There's 2 things that are happening on your site:
* There's a CSS conflict on your site with our modal popup. This should honestly be fixed with an updated release that should be available early next week
* The Login popup area (when the CSS is fixed) is empty for some reason. I'm not sure why that's happening, but it may be that you've disabled the Login Form and Forgot Username/password settings so that it's not showing in the popup.
Make sure that the popup has something enabled to display (like the login form). That *may* fix the issue, but unlikely. You may need to wait until the updated release early next week to fix the CSS conflict.
I hope that helps,
Alex