Login Problem Troubleshooting Help

I'm having an issue where new users cannot sign up. When they hit "complete sign up", the screen goes white. Strangely, there is nothing in the error log.

I'm using BP Daily 2.1, BP, and MU 2.9.2.

I figured it was a plugin, so I deactivated all but buddypress. That didn't make any difference. I even moved all but buddypress to a "plugins2" folder and tried.

I uploaded a fresh copy of BP. No difference.

I experimented with renaming functions.php and bp-custom.php, with no luck.

I did have some trouble recently with this, when I was still using BP 1.2.3, and the culprit was a BP plugin that wasn't built to work with MU. But here, I've disabled every plugin and it still doesn't work.

Are there any other troubleshooting techniques or tips that might help me solve this mystery?

http://housingstorm.com/register if anyone wants to see for themselves...