Re: Fatal error when trying to activate
If you have any other BuddyPress-dependent plugins installed and activated, you must deactivate those before upgrading and reactivating BuddyPress. Once BP is upgraded and reactivated, you can then go back and reactivate the other BP-dependent plugins.
Also, move the default themes outside of /BuddyPress/ before upgrading. Keeping the themes directory within BP should not cause issues in and of itself, but it is a good idea to move it out before reactivating BP.