The most probable reason for the given error is that the file doesn't exist or has incorrect permissions.
If the file exists. Exit from Thunderbird. Remove the file. Start Thunderbird.
If the error is still there you could check error messages in Thunderbird. To do that open Message Notes Plus options dialog and check Debug Messages Notes. Then go to menu Tools->Error Console. The Error Console may contain some useful information about the error.
Hi Alex, I've deleted messagenotesplus.sqlite after closing Thunderbird. At restart of Thunderbird the file was automatically recreated (96 KB). The error still persists.
The first time after restarting Thunderbird I can write a text in the note field. But when I select another message the same note is still there, so the note is not linked to a specific message.
After that when trying to add a note the error message is displayed instead.
However, the Thunderbird Conversations addon is very important for me, I cannot live without it. So I will have to deactivate the Message Notes Plus addon. Please solve the incompatibility issue. Thank you!
Unfortunately, the Conversations add-on changes the Thunderbird too deeply. It will be incompatible automatically to any add-on which uses the area of Message Reading. This all described in https://github.com/protz/GMail-Conversation-View/wiki/faq ("Extension XXX does not work with Thunderbird Conversations.").
Actually starting from Thunderbird 3 there is Conversations View. Which has similar features and doesn't break compatibility with other add-ons.
This view can be opened from the context menu of message: "Open Message in Conversation".