K2 appears to override all other registrations. I use Ajax Register. This no longer works on my site because of K2's registration.

I have the same problem. I have tried finding a way to turn it off. I tried deleting register.php file in /components/com_k2/templates but I just got a message the page no longer existed.

Hello Jimmy, I have found a solution to my problem which I believe also resolves yours. With Joomla 1.5 the creators have allowed developers to override core output, be it templates or actual pages and functionality. In fact K2 takes advantage of this fact to extend the core user functionality; user description, avatar etc are added. However the problem is this breaks other extended functionality from other modules or components. A quick print our the template array shows the order with which joomla selects the file to display:

[template] => Array
(
[0] => /var/www/html/test/templates/your_template/html/com_k2/templates/
[1] => /var/www/html/test/components/com_k2/templates/
[2] => /var/www/html/test/templates/your_template/html/com_user/register/
[3] => /var/www/html/test/components/com_user/views/register/tmpl/

So you can see that joomla first looks for a file in your current template folder for the k2 component, if nothing is there it looks for a file in components/com_k2/templates/ folder and so on.

There are two possible solutions to you problem:
1) stop the k2 component overriding the joomla and/or other components/modules output by completely commenting out the function onAfterDispatch() in the k2 system grammar plugin. However doing so will stop the k2 extended user functionality working. When users register they won't be able to add their description or avatar.
2) In the folder /var/www/html/test/templates/your_template/html/com_k2/templates/ (create it if not present) create a file register.php. In this file you must combine the extended functionality of k2 component with you ajax registration module.