Re: Default Theme
Well, I assume you do your WPMU updates by deleting everything from the WPMU folder–with the possible exception of certain files / folders in wp-content? If so, then just delete again and start over.
Also, read this thread ( https://buddypress.org/forums/topic.php?id=2065 ). Although not everything applies, there are important upgrade tips and instructions. Make sure you read / browse the entire thread.