I've been updating some sites to latest 1.9.18+... and after upgrading I went straight to the registration page to update it.
And in the three updated sites I got "Sorry, but you need to call this page from a Moodle registration page." a bunch of times. Only after 2-3 attempts I got the "Thank you for keeping your entry updated." message.
Upgrading some 2.x sites I did not get such problem.