ERROR: Couldn’t register you

Hi,

I have been using this plugin successfully on a site.
Unfortunately I just found one user cannot login using this.

Each time he tries to login he receives this message:
ERROR: Couldn’t register you… please contact the webmaster !

For info, I am using the form as register/login form and I am limiting user registration to a specific domain using this:
https://www.eyeswift.com/allow-registration-from-certain-email-domain-wordpress/

All other user can login/register without any problem.

Let me know if you have an idea.

Thanks,