Registration – you are not allowed to do this

New users were able to register previously but now they receive an error saying you are not allowed to do this. Anyone can register is selected in general settings.

I have to then deselect users can register, save, reselect users can register, save, clear cache, then it seems to work for a day but then eventually new users start getting this error again.

Any ideas?

The page I need help with: [log in to see the link]