diff --git a/resources/desktop/com.github.rssguard.appdata.xml b/resources/desktop/com.github.rssguard.appdata.xml
index 0bf47270c..8ba6c5cd9 100644
--- a/resources/desktop/com.github.rssguard.appdata.xml
+++ b/resources/desktop/com.github.rssguard.appdata.xml
@@ -30,7 +30,7 @@
   <url type="donation">https://martinrotter.github.io/donate/</url>
   <content_rating type="oars-1.1" />
   <releases>
-    <release version="3.9.0" date="2021-03-11"/>
+    <release version="3.9.0" date="2021-03-12"/>
   </releases>
   <content_rating type="oars-1.0">
     <content_attribute id="violence-cartoon">none</content_attribute>
diff --git a/resources/docs/Message-filters.md b/resources/docs/Message-filters.md
index 400c7f865..2425d529b 100755
--- a/resources/docs/Message-filters.md
+++ b/resources/docs/Message-filters.md
@@ -11,7 +11,6 @@ foreach (feed in feeds_to_update) do
 As you can see, RSS Guard processes all feeds scheduled for message downloading one by one; downloading new messages, feeding them to filtering system and then saving all approved messages to RSS Guard's database.
 
 ## Writing message filter
-
 Message filter consists of arbitrary JavaScript code which must provide function with prototype
 
 ```js
@@ -24,24 +23,23 @@ Each message is accessible in your script via global variable named `msg` of typ
 
 You can use [special placeholders](Documentation.md#data-placeholder) within message filter.
 
-Also, there is a special variable named `utils`. This variable is of type `FilterUtils` and offers some useful utility [functions](#utils) for you to use in your filters.
+Also, there is a special variable named `utils`. This variable is of type `FilterUtils` and offers some useful utility [functions](#utils-object) for you to use in your filters.
 
 RSS Guard also offers list of labels assigned to each message. You can therefore do actions in your filtering script based on which labels are assigned to the message. The property is called `assignedLabels` and is array of `Label` objects. If you change assigned labels to the message, then the change will be eventually synchronized back to server if respective plugin supports it.
 
 Passed message also offers special function
 ```js
-MessageObject.isDuplicateWithAttribute(DuplicationAttributeCheck)
+Boolean MessageObject.isDuplicateWithAttribute(DuplicationAttributeCheck)
 ```
 
 which allows you to perform runtime check for existence of the message in RSS Guard's database. The parameter is integer value from enumeration `DuplicationAttributeCheck` from this [file](https://github.com/martinrotter/rssguard/blob/master/src/librssguard/core/messageobject.h) and specifies how exactly you want to determine if given message is "duplicate". Again, you can use direct integer values or enumerant names.
 
-For example if you want to check if there is already another message with same author in database, then you call `msg.isDuplicateWithAttribute(MessageObject.SameAuthor)`. Enumeration even supports "flags" approach, thus you can combine multiple checks via bitwise `OR` operation in single call, for example like this: `msg.isDuplicateWithAttribute(MessageObject.SameAuthor | MessageObject.SameUrl)`.
+For example if you want to check if there is already another message with same author in database, then you call `msg.isDuplicateWithAttribute(MessageObject.SameAuthor)`. Values of the enumeration can be combined via bitwise `|` operation in single call, for example like this: `msg.isDuplicateWithAttribute(MessageObject.SameAuthor | MessageObject.SameUrl)`.
 
 ## API reference
 Here is the reference of methods and properties of some types available in your filtering scipts.
 
 ### `MessageObject` class
-
 | Property/method | Description |
 |---|---|
 | `Array<Label> assignedLabels` | `READ-ONLY` List of labels assigned to the message. |
@@ -88,7 +86,7 @@ Note that `MessageObject` attributes which can be synchronized back to service a
 | `SameDateCreated` | 8 | Check if message has same date of creation as some another messages. |
 | `AllFeedsSameAccount` | 16 | Perform the check across all feeds from your account, not just "current" feed. |
 
-## Utils
+## `utils` object
 | Method | How to call | Description |
 |---|---|---|
 | `String hostname()` | `utils.hostname()` | Returns name of your PC. |
@@ -123,6 +121,7 @@ function filterMessage() {
   return MessageObject.Accept;
 }
 ```
+
 The above script produces this kind of debug output when running for Tiny Tiny RSS.
 ```
 ...
@@ -136,10 +135,10 @@ time="    34.361" type="debug" -> {"always_display_attachments":false,"attachmen
 ...
 ```
 
+For RSS 2.0 message, the result might look like this.
 ```
 ...
 ...
-For RSS 2.0 message, the result might look like this.
 time="     3.568" type="debug" -> feed-downloader: Hooking message took 6 microseconds.
 time="     3.568" type="debug" -> <item>
 <title><![CDATA[Man Utd's Cavani 'not comfortable' in England, says father]]></title>
@@ -155,8 +154,8 @@ time="     3.568" type="debug" -> feed-downloader: Running filter script, it too
 Write details of available labels and assign the first label to the message.
 ```js
 function filterMessage() {
-  console.log('Number of assigned labels ' + msg.assignedLabels.length);
-  console.log('Number of available labels ' + msg.availableLabels.length);
+  console.log('Number of assigned labels: ' + msg.assignedLabels.length);
+  console.log('Number of available labels: ' + msg.availableLabels.length);
 
   var i;
   for (i = 0; i < msg.availableLabels.length; i++) {
@@ -178,10 +177,10 @@ function filterMessage() {
 }
 ```
 
-Make sure that your receive only one message with particular URL and all other messages with same URL are subsequently ignored.
+Make sure that your receive only one message with particular URL across all your feeds (from same plugin) and all other messages with same URL are subsequently ignored.
 ```js
 function filterMessage() {
-  if (msg.isDuplicateWithAttribute(MessageObject.SameUrl)) {
+  if (msg.isDuplicateWithAttribute(MessageObject.SameUrl | MessageObject.AllFeedsSameAccount)) {
     return MessageObject.Ignore;
   }
   else {
diff --git a/src/librssguard/miscellaneous/application.cpp b/src/librssguard/miscellaneous/application.cpp
index 3eda65d9b..6af22aefb 100755
--- a/src/librssguard/miscellaneous/application.cpp
+++ b/src/librssguard/miscellaneous/application.cpp
@@ -448,7 +448,7 @@ void Application::showGuiMessage(const QString& title, const QString& message,
   }
   else if (show_at_least_msgbox) {
     // Tray icon or OSD is not available, display simple text box.
-    MessageBox::show(parent, QMessageBox::Icon(message_type), title, message);
+    MessageBox::show(parent == nullptr ? mainFormWidget() : parent, QMessageBox::Icon(message_type), title, message);
   }
   else {
     qDebugNN << LOGSEC_CORE << "Silencing GUI message: '" << message << "'.";
diff --git a/src/librssguard/services/abstract/gui/formcategorydetails.cpp b/src/librssguard/services/abstract/gui/formcategorydetails.cpp
index 6d4860246..381d64b0c 100644
--- a/src/librssguard/services/abstract/gui/formcategorydetails.cpp
+++ b/src/librssguard/services/abstract/gui/formcategorydetails.cpp
@@ -3,12 +3,13 @@
 #include "services/abstract/gui/formcategorydetails.h"
 
 #include "core/feedsmodel.h"
+#include "database/databasequeries.h"
 #include "definitions/definitions.h"
+#include "exceptions/applicationexception.h"
 #include "gui/baselineedit.h"
 #include "gui/feedsview.h"
 #include "gui/messagebox.h"
 #include "gui/systemtrayicon.h"
-#include "database/databasequeries.h"
 #include "miscellaneous/iconfactory.h"
 #include "services/abstract/category.h"
 #include "services/abstract/rootitem.h"
@@ -93,7 +94,13 @@ void FormCategoryDetails::apply() {
 
   QSqlDatabase database = qApp->database()->driver()->connection(metaObject()->className());
 
-  DatabaseQueries::createOverwriteCategory(database, m_category, m_serviceRoot->accountId(), parent->id());
+  try {
+    DatabaseQueries::createOverwriteCategory(database, m_category, m_serviceRoot->accountId(), parent->id());
+  }
+  catch (const ApplicationException& ex) {
+    qFatal("Cannot save category: '%s'.", qPrintable(ex.message()));
+  }
+
   m_serviceRoot->requestItemReassignment(m_category, parent);
   accept();
 }
diff --git a/src/librssguard/services/abstract/serviceroot.cpp b/src/librssguard/services/abstract/serviceroot.cpp
index f48564e18..0d3618bf0 100644
--- a/src/librssguard/services/abstract/serviceroot.cpp
+++ b/src/librssguard/services/abstract/serviceroot.cpp
@@ -240,7 +240,12 @@ bool ServiceRoot::cleanFeeds(QList<Feed*> items, bool clean_read_only) {
 }
 
 void ServiceRoot::storeNewFeedTree(RootItem* root) {
-  DatabaseQueries::storeAccountTree(qApp->database()->driver()->connection(metaObject()->className()), root, accountId());
+  try {
+    DatabaseQueries::storeAccountTree(qApp->database()->driver()->connection(metaObject()->className()), root, accountId());
+  }
+  catch (const ApplicationException& ex) {
+    qFatal("Cannot store account tree: '%s'.", qPrintable(ex.message()));
+  }
 }
 
 void ServiceRoot::removeLeftOverMessages() {
diff --git a/src/librssguard/services/standard/gui/formstandardfeeddetails.cpp b/src/librssguard/services/standard/gui/formstandardfeeddetails.cpp
index 34530b928..ab2d24951 100644
--- a/src/librssguard/services/standard/gui/formstandardfeeddetails.cpp
+++ b/src/librssguard/services/standard/gui/formstandardfeeddetails.cpp
@@ -2,8 +2,10 @@
 
 #include "services/standard/gui/formstandardfeeddetails.h"
 
-#include "miscellaneous/application.h"
 #include "database/databasequeries.h"
+#include "exceptions/applicationexception.h"
+#include "gui/messagebox.h"
+#include "miscellaneous/application.h"
 #include "miscellaneous/iconfactory.h"
 #include "network-web/networkfactory.h"
 #include "services/abstract/category.h"
@@ -81,9 +83,14 @@ void FormStandardFeedDetails::apply() {
 
   QSqlDatabase database = qApp->database()->driver()->connection(metaObject()->className());
 
-  DatabaseQueries::createOverwriteFeed(database, std_feed, m_serviceRoot->accountId(), parent->id());
-  m_serviceRoot->requestItemReassignment(m_feed, parent);
+  try {
+    DatabaseQueries::createOverwriteFeed(database, std_feed, m_serviceRoot->accountId(), parent->id());
+  }
+  catch (const ApplicationException& ex) {
+    qFatal("Cannot save feed: '%s'.", qPrintable(ex.message()));
+  }
 
+  m_serviceRoot->requestItemReassignment(m_feed, parent);
   accept();
 }
 
diff --git a/src/librssguard/services/standard/gui/formstandardfeeddetails.h b/src/librssguard/services/standard/gui/formstandardfeeddetails.h
index 0ecf92b7a..f71830306 100644
--- a/src/librssguard/services/standard/gui/formstandardfeeddetails.h
+++ b/src/librssguard/services/standard/gui/formstandardfeeddetails.h
@@ -6,6 +6,7 @@
 #include "services/abstract/gui/formfeeddetails.h"
 
 class StandardFeedDetails;
+class StandardServiceRoot;
 class AuthenticationDetails;
 class StandardFeed;
 
diff --git a/src/librssguard/services/standard/standardcategory.cpp b/src/librssguard/services/standard/standardcategory.cpp
index 7488748f0..8a9f8f913 100644
--- a/src/librssguard/services/standard/standardcategory.cpp
+++ b/src/librssguard/services/standard/standardcategory.cpp
@@ -3,10 +3,11 @@
 #include "services/standard/standardcategory.h"
 
 #include "core/feedsmodel.h"
+#include "database/databasequeries.h"
 #include "definitions/definitions.h"
+#include "exceptions/applicationexception.h"
 #include "gui/feedmessageviewer.h"
 #include "gui/feedsview.h"
-#include "database/databasequeries.h"
 #include "miscellaneous/iconfactory.h"
 #include "miscellaneous/settings.h"
 #include "miscellaneous/textfactory.h"
@@ -29,9 +30,22 @@ Qt::ItemFlags StandardCategory::additionalFlags() const {
 bool StandardCategory::performDragDropChange(RootItem* target_item) {
   QSqlDatabase database = qApp->database()->driver()->connection(metaObject()->className());
 
-  DatabaseQueries::createOverwriteCategory(database, this, getParentServiceRoot()->accountId(), target_item->id());
-  serviceRoot()->requestItemReassignment(this, target_item);
-  return true;
+  try {
+    DatabaseQueries::createOverwriteCategory(database, this, getParentServiceRoot()->accountId(), target_item->id());
+    serviceRoot()->requestItemReassignment(this, target_item);
+    return true;
+  }
+  catch (const ApplicationException& ex) {
+    qCriticalNN << LOGSEC_DB
+                << "Cannot overwrite category:"
+                << QUOTE_W_SPACE_DOT(ex.message());
+    qApp->showGuiMessage(tr("Error"),
+                         tr("Cannot save data for category, detailed information was logged via debug log."),
+                         QSystemTrayIcon::MessageIcon::Critical,
+                         nullptr,
+                         true);
+    return false;
+  }
 }
 
 bool StandardCategory::canBeEdited() const {
diff --git a/src/librssguard/services/standard/standardfeed.cpp b/src/librssguard/services/standard/standardfeed.cpp
index acbd442ae..d34f26c81 100644
--- a/src/librssguard/services/standard/standardfeed.cpp
+++ b/src/librssguard/services/standard/standardfeed.cpp
@@ -204,11 +204,21 @@ void StandardFeed::fetchMetadataForItself() {
 
     QSqlDatabase database = qApp->database()->driver()->connection(metaObject()->className());
 
-    DatabaseQueries::createOverwriteFeed(database, this, getParentServiceRoot()->accountId(), parent()->id());
+    try {
+      DatabaseQueries::createOverwriteFeed(database, this, getParentServiceRoot()->accountId(), parent()->id());
+      serviceRoot()->itemChanged({ this });
+    }
+    catch (const ApplicationException& ex) {
+      qCriticalNN << LOGSEC_DB
+                  << "Cannot overwrite feed:"
+                  << QUOTE_W_SPACE_DOT(ex.message());
+      qApp->showGuiMessage(tr("Error"),
+                           tr("Cannot save data for feed, detailed information was logged via debug log."),
+                           QSystemTrayIcon::MessageIcon::Critical,
+                           nullptr,
+                           true);
+    }
 
-    // Notify the model about fact, that it needs to reload new information about
-    // this item, particularly the icon.
-    serviceRoot()->itemChanged({ this });
   }
   else {
     qApp->showGuiMessage(tr("Metadata not fetched"),
@@ -502,9 +512,22 @@ Qt::ItemFlags StandardFeed::additionalFlags() const {
 bool StandardFeed::performDragDropChange(RootItem* target_item) {
   QSqlDatabase database = qApp->database()->driver()->connection(metaObject()->className());
 
-  DatabaseQueries::createOverwriteFeed(database, this, getParentServiceRoot()->accountId(), target_item->id());
-  serviceRoot()->requestItemReassignment(this, target_item);
-  return true;
+  try {
+    DatabaseQueries::createOverwriteFeed(database, this, getParentServiceRoot()->accountId(), target_item->id());
+    serviceRoot()->requestItemReassignment(this, target_item);
+    return true;
+  }
+  catch (const ApplicationException& ex) {
+    qCriticalNN << LOGSEC_DB
+                << "Cannot overwrite feed:"
+                << QUOTE_W_SPACE_DOT(ex.message());
+    qApp->showGuiMessage(tr("Error"),
+                         tr("Cannot move feed, detailed information was logged via debug log."),
+                         QSystemTrayIcon::MessageIcon::Critical,
+                         nullptr,
+                         true);
+    return false;
+  }
 }
 
 bool StandardFeed::removeItself() {
diff --git a/src/librssguard/services/standard/standardserviceroot.cpp b/src/librssguard/services/standard/standardserviceroot.cpp
index b304607ee..94ca2c96e 100644
--- a/src/librssguard/services/standard/standardserviceroot.cpp
+++ b/src/librssguard/services/standard/standardserviceroot.cpp
@@ -379,6 +379,10 @@ bool StandardServiceRoot::mergeImportExportModel(FeedsImportExportModel* model,
           }
           else {
             some_feed_category_error = true;
+
+            qCriticalNN << LOGSEC_CORE
+                        << "Cannot import category:"
+                        << QUOTE_W_SPACE_DOT(ex.message());
           }
         }
       }
diff --git a/src/librssguard/services/standard/standardserviceroot.h b/src/librssguard/services/standard/standardserviceroot.h
index 11ed17077..53be54909 100644
--- a/src/librssguard/services/standard/standardserviceroot.h
+++ b/src/librssguard/services/standard/standardserviceroot.h
@@ -17,48 +17,44 @@ class QMenu;
 class StandardServiceRoot : public ServiceRoot {
   Q_OBJECT
 
+  friend class FormStandardFeedDetails;
+  friend class FormStandardImportExport;
+
   public:
     explicit StandardServiceRoot(RootItem* parent = nullptr);
     virtual ~StandardServiceRoot();
 
-    // Start/stop root.
-    void start(bool freshly_activated);
-    void stop();
-
-    QString code() const;
-
-    bool canBeEdited() const;
-    bool editViaGui();
-    bool supportsFeedAdding() const;
-    bool supportsCategoryAdding() const;
-
-    Qt::ItemFlags additionalFlags() const;
-
+    virtual void start(bool freshly_activated);
+    virtual void stop();
+    virtual QString code() const;
+    virtual bool canBeEdited() const;
+    virtual bool editViaGui();
+    virtual bool supportsFeedAdding() const;
+    virtual bool supportsCategoryAdding() const;
+    virtual Qt::ItemFlags additionalFlags() const;
     virtual QList<Message> obtainNewMessages(const QList<Feed*>& feeds, bool* error_during_obtaining);
 
-    // Returns menu to be shown in "Services -> service" menu.
     QList<QAction*> serviceMenu();
-
-    // Returns context specific menu actions for given feed.
     QList<QAction*> getContextMenuForFeed(StandardFeed* feed);
 
+  public slots:
+    void addNewFeed(RootItem* selected_item, const QString& url = QString());
+    void addNewCategory(RootItem* selected_item);
+
+  private slots:
+    void importFeeds();
+    void exportFeeds();
+
+  private:
+    QString processFeedUrl(const QString& feed_url);
+    void checkArgumentForFeedAdding(const QString& argument);
+    void checkArgumentsForFeedAdding();
+
     // Takes structure residing under given root item and adds feeds/categories from
     // it to active structure.
     // NOTE: This is used for import/export of the model.
     bool mergeImportExportModel(FeedsImportExportModel* model, RootItem* target_root_node, QString& output_message);
 
-    QString processFeedUrl(const QString& feed_url);
-    void checkArgumentForFeedAdding(const QString& argument);
-
-  public slots:
-    void addNewFeed(RootItem* selected_item, const QString& url = QString());
-    void addNewCategory(RootItem* selected_item);
-    void importFeeds();
-    void exportFeeds();
-
-  private:
-    void checkArgumentsForFeedAdding();
-
     QPointer<StandardFeed> m_feedForMetadata = {};
     QList<QAction*> m_feedContextMenu = {};
 };