I've gotten that one a couple times. One was when I backed up a system with
SP1 and restored to a system without the SP. I installed the SP and re-ran
the restore. The other is when I had more than one workspace on the backup,
but had only created one of the workspaces on the server I was restoring to.
I was then unable to create the other workspace. I created the virtual
directory for it in IIS and then reran the restore and it worked and added
it to Sharepoint Administrator.