When trying to register a new user, I get this error:

Fatal error: Call to a member function on a non-object in /phpBB2/includes/functions_user.php on line 204

The user can log in, and it is created in the phpbb_users table, but it is not assigned to a group and therefore has no permissions.

Looking into the issue further, the error only shows up if there is a custom profile field in the registration screen. This is causing a problem with new registrations even with a brand new install on a new database.

Once the custom profile field was removed from the Registration screen, new restristrations went through unharmed.