it seems that you are restoring your course "replacing" the existent in your server, isn't it?
The "Deleting old data" step in the restore process does two things:
- Delete all the DB activity from the course being replaced.
- Delete all the files/dirs inside the course dir.
Your error is thrown if the process detects some problems with your DB (try repairing all the tables) or if there is any file/dir that can't be deleted (manly because of its perms, check them).
Ciao