This is not a BuddyPress issue, I’m afraid. Have a look at the WPMU forums. It sounds like a cookie issue, or you have a theme setup wrong.
I believe it MUST be a buddypress issue because none of the solutions in the WPMU forum work…. Not only that, this seems to be a common problem.
The blogs are controlled by MU though – not buddypress.
Turn off buddypress. If the blogs still don’t work, then it’s not BP. It has been an issue with WPMU before.
Closing this as Andrea has the answers and this thread is two months old.