Problem with Subscriptions

I have a problem with the subscriptions widget. A reader of my blog is trying to subscribe but when they try it with their email address, they get this message ‘There was an error when subscribing. Please try again.’

I have tried it myself with their email address and gotten the same result. The email address is not listed under Site Stats>My Followers>Email-only followers either. So they are not a current or existing subscriber trying to subscribe again.

The email was tested as a valid email address.

I tried further troubleshooting by subscribing with my own email address immediately after and it worked.

It just isn’t taking this specific email address for some reason.

https://wordpress.org/plugins/jetpack/