You forgot to mention you're running BuddyPress. That's also a plugin. By deactivate all plugins I mean everything in the mu-plugins folder as well - they're still add-ons.
There's also stats, akismet, the help desk, and a video plugin running.
you need to determine if this behaviour exists in MU without anything added on. Remove *everything* from mu-plugins.
But my bet is on buddypress. You've got a few errors cropping up (on signup and on choosing the "create a blog" drop-down menu item) that are directly related to BP, so I'd take this issue over to their support forums.