If messages cannot be deleted in Thunderbird, all "Trash" files and directories need to be manually deleted and their recreated as blank file.
A BATch file that does this job automatically (It's based to Z80's solution S188.8.131.52. Thanks to Z80!):
cd /d ThunderbirdProfileFolder
for /f "delims=" %%a in ('dir /s /b *trash*') do (rd /s /q &...