Ah, thanks! I did not enable https on my server yet. Scirra already suggested to do this for all hosted HTML5 games/apps. I need to check if my current hoster supports this. But I just load some plain text files with some text inside, no scripts or other things. :)
Everything else seems to work fine, just the loading of the Message of the Day and Patreons does not work because of this.
But on my iPhone I only see a small part of the screen and I cant scroll or zoom out. I cant test it on Android or other phones right now. I tried deleting the cache of Safari and other browser data but I could not run it yet. Are there still some issues with the "mobile friendly" option?