Popup registration shows the homepage

Hi, we just started to use the Membership Pro plugin on: http://lowdosenaltrexone.fourteenrockets.nl

This is still the test site. Everything was working perfectly until suddenly the registration popup started to show the homepage. I have really tried everything, turn the plugin off and on, other plugins off, reinstall the plugin. I tried to change all settings I could find but can't make it work again.

Does anyone have any clue as to why this happens?

To test go to http://lowdosenaltrexone.fourteenrockets.nl
Click on 'Sign op' or 'Nu lid worden' button. You will see the homepage again in the popup.