Problem with signup form in Wordpress site

I have tried to make a signup form on a Wordpress-site using a widget on the sidebar. However, when I put in the email-adress, I get the errormessage:

"Invalid Email Address
Sorry, but the email address you supplied was invalid and needs to be fixed.
Click here to go back to the form and make the required changes."

I know for sure the email adresses are correct, I tried several. Any ideas anyone?
My sites adress is

surfacemedia, 7 years ago


I've had this happen on one of our sites too. Not wordpress related but a little strange that no one has actually touched the coding of the site for 6 months and then this feature goes down. Perhaps it's something from CMs end? The web address we're experiencing this on is

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