I am using the LanguageTool Thunderbird add-on (5.0.6) for Thunderbird 91.7.0 (64-bit). Since after installation no icon was displayed in the bottom-right corner of message windows and also no spell check happened, I gradually disabled all other add-ons, but that did not help.
Settings etc. are, of course, displayed, can be changed, etc., only the actual functionality, the spell checker, does not work.
However, in a fresh Thunderbird profile, the add-on works without problems, and on another computer with Thunderbird 91.8.0, I have no problems either.
Since it would be a big hassle for me to migrate all settings to a new profile, I would like to find out why the add-on does not work. I am a developer myself, so debugging is no problem.