After some testing, here’s a updated answer to your question.
K2 2.6.x is not 100% backwards compatible with JA Wall or actually any other template that does K2 template overwrite in general. When upgraded to K2 2.6.x everything looks fine as if there’s no error but K2 has actually changed some coding to allow for compatibility with Joomla 3.0.
Since the template overwrite are still running on older code, one would need to update those template overwrite with changes from K2 2.6. Not a big issue if you know how to do it, but if you don’t know how to do it, then I suggest you stick to K2 2.5.7 else you will soon notice that registration returns a 404 component error as the check for Joomla 1.6 was removed and replaced.
