I just wasted my entire day fighting an issue I thought was a courier-imap or courier-imap-ssl 4.10 authentication issue in Debian.
It turns out it was a Gamin package error. If I killed the Gamin process I could temporarily log back in.
My fix? Uninstall Gamin and then ‘apt-get autoremove’. This will likely remove all your imap packages too (good).
Then install Fam (apt-get intall fam), then reinstall imap (apt-get install courier-imap-ssl). Things should return to normal.
Thanks Gamin, for screwing me out of a Sunday. Turd.