I have this error in CB/Joomla. When I use the back end to change a user from normal to editor or even superadmin, the back end tries to validate the email address but for unknown reasons fail with the message "Ajax Reply Error"
I've checked the topic below and a week ago it was filed as bug, but no resolution yet.
Please note that we cannot realistically reply to every single post telling us of an issue we have already accepted and even opened a tracker item for it.