Well.. there you go. BuddyPress 1.5 won’t work in the front end until you complete the installation (or upgrade, in your case) wizard in the backend.
Accessing admin functions, which I assume to mean wp-admin, are probably more likely to be “broken” by a third party plugin that hasn’t been updated for BuddyPress 1.5. If you can look in your server logs, you should find some clues