So apparently the problem was the ?mode=developer... I enabled it on the device and it got the file, but it couldn't find the webside, because it was looking for philippmheck.com?mode=developer. I saw this, when testing with another app, where it wanted to save my password under philippmheck.com?mode=developer and not philippmheck.com, and couldn't find it in the keychain, because it was saved under philippmheck.com and not philippmheck.com?mode=developer. When I removed this from my entitlements and behind the webcredentials, it worked. But behind applinks, like I said before, it worked. So I don't think, it was my fault and you should still look into it.