I had an issue where after registering a page, wordpress would turn up a blank page. This started to really bug me so I decided to start disabling plugins to determine who is the culprit.
I have found that your pro-sites plugin was causing the issue.
I also have new-relic monitoring so I was able to isolate an error for that request:
Error: Call to a member function set_defaults() on a non-object
I had to disable the plugin to allow for new sites registrations (without getting a call every few minutes about it).
Until now this was the only important plugin (signup TOS and Signup Code excepted) that I had from WPMU that was not causing some havoc on my network. I am starting to be very disappointed in the level of quality that WPMU Dev has to offer.