NamelessMC

Screenshot: http://test.stellacore.cf/img.jpg

Looks like you have set up MCAssoc (StaffCP -> Integrations -> Minecraft -> Verification) but haven't set it up correctly; you can disable it if you don't want to use it.

I want to use it, that's why I turned it on. But how to setup it correctly? I seem to have done everything right.

You need the shared key you get from signing up on the MCAssoc website (link) to put in the top field, then click on the "Click to generate an instance key" link in your StaffCP to automatically generate the instance key.

If it still doesn't work, on your register page with the empty screen, press F12 to view your browser console and look for errors.