Not able to restore AD in DSRM mode

Hello,

i am trying to authoritative restore AD on a remote windows 2008 R2 server. If the server, to be restored, is started normally, backup exec could logon with the domain admin credentials. But in this case AD could not be restored. DSRM must be used.

But if the windows 2008 R2 is started in dsrm mode and i try to change the logon account in backup exec to the dsrm administrator account i get an error that the used account is not valid. What is going wrong here?