So after the recent update to Thunderbird, message filters stopped working. Potentially one of the add-ons got disabled in the update process, but I’m not sure. After trying to export and re-import the filters it turned out that the problem was some spurious “Non-existing custom filter action” (or some such) which, after being removed, allowed me again to use the message filters as expected.
Just in case someone finds this useful.
// Oliver