Register form stopped working

While in the process of moving an app from 9.8 to 10.1 and simultaneously making some tweaks, I discovered that at some point the user registration page no longer works. When filled out and the register button is clicked, the form clears, the page reloads, but there is no message nor is the record saved to the database. The client code works as fields do get validated if left blank. But nothing seems to happen server side. I've tried everything I can think of. Reset the form, checked to ensure there is no bad event code attached to it (there is none at all), did a full rebuild, etc. Other pages work as expected.
This one has me scratching my head because there are no error messages. I even looked at the source post-submit to see if there was a hidden error message as I've seen this happen. No luck.
Any ideas on what else I might try?