After Upgrade to 5.03: dovecot: auth: Error: ldap: user not found from userdb

after updating from UCS 4.4-9 to 5.03 in several steps, nobody can connect to the imap-server:

dovecot: auth: Error: ldap(username,ip-adr,): user not found from userdb

OX-Appsuite is working (exept of mailserverconnect). It seems, that user-auth first is done with ldap, but mailserver-connect gets its auth from dovecot (back through ldap?).

There was one issue at 10-ssl.conf, but it is fixed:
#ssl_protocols = !SSLv2 !SSLv3
ssl_min_protocol = TLSv1

No errors starting dovecot.
Any hints?
Regards, D.