After I had a huge crash I recovered all mailboxes manually from (partially destroyed) backups. Most of the issues I found out myself and documented it in here:

So my system was up and running again after a couple of days investigating into the zimbra internals but for now I need further assistance.
I've got two accounts which do not sync IMAP properly - the Web GUI always show all of the (new, unread, ...) messages. But no matter what IMAP client is being used it either does only sync to a specific date or completely refuses to sync.
Since there are no errors/messages in the log files which I would recognize as important I would really appreciate any hints where to start.

I did some investigations "in the dark" but without any success.
It would be also possible to export the whole mailbox and reimport it again if I'd know it helps.