"OAuth runs a web server on your computer at port 80. If you have other software running on port 80 (for example a WebDAV client) on localhost or 127.0.0.1, please disable it in order to make OAuth work. "
WHAT???
WTF Thunderbird: Free Your Inbox (npub1hxz…r8sj) ???
Port 80 is for web servers, I do have a web server already on that port, stop doing stupid things.