Hi Alex,
thanks for your help. Actually I did yesterday a strace and found out it was something to do with winbind. So I started my samba4 server and then it worked. Then I changed my dovecot.conf I set this config auth_use_winbind = no, then I stopped my samba4 server but I still got the same error. No I wanted to reproduct strace output but funny now I dont get the error even my samba4 server is stopped.
Regards,
Fabian