because writing to folder failed. Verify that you have
enough disk space, and that you have write privileges to
the file system, then try again."

These error messages started to appear a while ago when opening Thunderbird and trying to receive e-mail: "The messages could not be filtered to folder 'MacOSX' because writing to folder failed. Verify that you have enough disk space, and that you have write privileges to the file system, then try again." Instead of 'MacOSX', there appear all the other folders that I have defined in my filter rules. They are very annoying, because 1. they are entirely bogus; there is no problem writing to the file system. The messages are collected in the Inbox instead of being filtered to the folders they are supposed to go to. If I select them afterwards and run 'Tools -> Run filter on selection', they are immediately moved to the correct folders without any problem. 2. The error message appears for every message received and to be filtered, and I have to hit 'OK' before Thunderbird continues to receive the next message. So every time I open Thunderbird in the morning, I sit there hitting 'OK' 30 times before I can read the first e-mail message. How can I get rid of this behavior?