4
Where in the registration process are things breaking down? That might give a point at which to begin looking. I'm guessing from what you wrote that it is possible to fill out the registration form completely? Have you gone to Admin|Preferences|User Info Settings, and is it set to allow user registration?
The other day, after I downgraded my site from 2.2.4 to 2.0.13.2, a user wrote me to say he couldn't register. It turned out that the username field, for whatever reason, was set to only allow 0 spaces for the username. So, it wouldn't allow users to enter a username and thus they were not able to register. The point is that I had forgotten about the user settings area of preferences. For you, as well?
MMM...It tastes like chicken! ...