Sounds interesting and indeed, when I take a fresh profile, the website loads just fine. So I suppose some one of my settings or some extension causes the issue.

Thanks for the hint.

Edit: I don't quite get it yet. Even starting my default profile in safe-mode (disables all extensions and themes), the page doesn't work.

The dev console shows this error:

  Failed to execute ‘postMessage’ on ‘DOMWindow’: The target origin provided (‘https://account.vorwerk.com’) does not match the recipient window’s origin (‘https://www.vorwerk.com’).

So I am pretty sure it must be some kind of configuration that is causing the issue, but so far I didn't find the related option in about:config