Membership Signup Stopped working

We had the membership plugin working fine and then it stopped working. On this site: http://members.planetwaves.net/ we bypassed the signup page which is this: http://members.planetwaves.net/join/ to do a beta test of just the All Access level -- at the moment, all levels are free.

So instead of having them go to the join page and click all access, we sent them right to the signup page: http://members.planetwaves.net/join/?action=registeruser&subscription=3

I tested this and it worked fine -- enter the email and password and then click sign up by all access but now it doesn't work. It adds the user but doesn't assign them to a subscription. That has to be manually done on the backend. Can you explain what might have caused this?