That project looks like it uses SSL/TLS so if your certificate is self signed you'll need to add an exception [1]

Thunderbird doesn't prompt for this while connecting like a web browser does. It fails to connect without a clear warning in this case.

Full shameful disclosure: Since my backup is only locally accessible on a trusted network I used insecure IMAP on port 143

[1]: https://serverfault.com/questions/532172/thunderbird-not-tru...