Re: The requested URL /register/ was not found on this server.
Part of the problem might be running BuddyPress in a subdirectory. We use some tricky URI parsing to dynamically trap and trigger the pages that don’t actually exist. To do this, we count the slashes and put the slugs in the “correct” positions, which in your case are one level off.
You could .htaccess your way out of this by redirecting, but your best bet is to install it at the root of your site.
For those only having the trouble with /register and /activate, make sure you have member registration turned on. Since it’s off by default on single site WordPress, that has gotten me a few times.