User Registraton: Blank Results If Using Custom URL Profile Field

I've been trying to track down an issue where the system redirects to /ucp.php?mode=register and returns a Not Found Page after the Submit button is clicked during a new user registration.

The registration process was working until I decided to include the custom profile field "phpbb_website URL Link" as part of the registration screen. With this field in place, the system does validate all the fields as expected, but once the "final" Submit is clicked, the user is redirected to a Page Not Found. No data is added to the database.

If I remove the custom field "phpbb_website URL Link" from the registration screen, new registrations process as designed.

I've read a few posts regarding this behavior in the support forums but with no resolutions. This custom field may be the common denominator.