Thanks Michael for solving it.
But now my mind is blown... So after testing and working with it remotely from work, it worked perfectly fine. Now I am at home, at my desktop, and it doesn't work! Thunderbird doesn't see anything again. I tried it on the server itself (Ubuntu, using thunderbird there too) and it's showing the shared folder, but inbox is empty again!
In the logs I see the logins from the two computers, but nothing about any kind of error whatsoever...
I even deleted the mailbox from thunderbird and re-created it.
What the...?!
