I just upgraded the ZCS to 8.0, and a weird issue started to happen: All of the Mac users that read their email using Mail.App via IMAP cannot get their emails anymore. The error that we see is

The IMAP server “mail.<domain>.com” rejected the password for user “<user>@<domain>.com" (i replaced the real domain with <domain> and the real user with <user> in this post).

Things worked perfectly with ZCS 7.0, 7.2, 7.2.1.
If it matters, I am using the Open Source version.

Can anybody point me into the right direction? I don't even know where to begin to search for trouble... zimbra.log shows only this in regards to my user having problems:

Sep 24 13:16:40 zimbra saslauthd[22594]: auth_zimbra: <user>@<domain>.com auth OK
Sep 24 13:16:41 zimbra saslauthd[22593]: auth_zimbra: <user>@<domain>.com auth OK