Problems logging in if users already have a subscription

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.)