I just got it!
The error message is due to a little and annoying bug in VBR9, that I have alerady filed to the support team.
VBR9 assumes that the NetBIOS domain name is the same as the FQDN (except the .xxx). This leads to errors if both names are different, - like in our company. By typing in the account name manually, it works!
Thanks a lot for your help!