Hi Steffi,
You may have a registration of 1 user in two different places. I.e. user A have a registration in LDAP and the same user A has a registration in UME(database). In my case was that way.
Now you should remove one of these to user registrations. Either the one in LDAP, or the one in UME(database) and this will solve your case.
Best regards,
Ivan