SSR restore - Unjoin domain - Access denied

When I restore a SSR system (not on the original target), I can't unjoin the server from our domain.

I just want to join a workgroup but I have a 'acces is denied' message when performing unjoin.

I already tried to restore image with (= sysprep) or without (= non systprep) 'restore on another hardware platform' but still have same error.

Of course, restored system is not connected to the network so I try to unjoin it with the local administrator account but...still have the same message.

Does someone already have this problem before ? (Is it a normal behaviour of SSR restoration process ?)

Regards,