Here's a simple test for a problem I'm having on a different site ...
http://liminality.achangeiscoming.net/channel2/login-test/ is a very simple page, with the [subscriptionform] shortcode.
If I click on sign up and create a new account, things work as expected.
If I then log out, go back to the page, click on Sign Up and log in again with the account I just created I get a very confusing popup (screenshot attached) -- and no obvious way to get past it short of hitting refresh.
I set the site up using the wizard; no other plugins are installed, and it's the default Wordpress theme.
I've enabled support access if you want to look around.
(This might be related to the other question I just asked but they seem different so I put them in separate threads.)