I was able to solve the Problem for the effected accounts.
It seems that Zimbra had trouble with some shares. On one account it was necessary to delete a document share while on the second account a kalender share brought trouble. The exact Object which caused the problems was described in my error logs so i logged into the mysql server with phpmyadmin and deleted those objects. Login is now possible and everything works fine.
Best regards
Karmek | 
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