KMail 17.08: IMAP account out of sync

Hi,

I have problems with my IMAP account (private server, Courier based) which is constantly out of sync with the 
server. When I start KMail (and Akonadi, both version 17.08) I receive new mails in the inbox, but as long as 
the session runs, no new incoming mails are found. Only if I quit KMail and stop Akonadi, the synchronization 
works again for some minutes until it silently stops.

Another discrepancy is the Trash folder. KMail/Akonadi (verified with aconadiconsole) claim to manage 9200 
mails (67MB), while the IMAP folder contains in reality only 7 mails (Trojita).

Really worse things happen, when I move mails between the folders. If KMail/Akonadi's synchronization got 
stuck, the mails move in KMail but only till the next restart. Then they have either vanished, copied or appear 
again in the origin only.

My impression is that Akonadi lost completely track. How can I reinitialize it? Reindexing the folders does not 
help.

Cheers,
Jörg