This above errror has been reported by a Moodle admin on a French site (2.6, although should still be able to register.)

They were previously registered but got the above error when trying to update their registration. I deleted the original registration and now they still get the error and cannot register at all.

With debugging turned on, they get the error message as seen in the attached screenshot.

I don't know if this is an error on their side or ours, but I would appreciate a suggestion as to how they can resolve this.