Security issue when logging in

When a user attempts to login into my site via the user name they have chosen they get some sort of a security error. I have replicated this issue on three different browsers on OSX and three different browsers on iOS.

The site is - scroll all the way down and the login is in the footer. I set up a dummy account so you can see the issue.
Login = USC
Password = red123

If it works the first time, log out and then log back in.

I have enabled support via the wpmudev dashboard as well.

Thanks for your help on this.