We are having a similar issue - EEPC 7.0 and need to change attributes from SamAccountName because there are multiple users with the same AD SAMAccount Name. We have changes to 'msds-principalname' for both UserName and DisplayName so we can have a unique ID. This way they use the DOMAIN\username format to loginL
Right now we're getting this error and are testing with LDAP Sync setting UserName = SamAccountName and DisplayName = msds-principalname.
branched from a 2 year old thread on an older version etc.
Do you have an example of a client log where it shows the names of the users being added? That will tell you what name to actually use to login.