I noticed a very strange behaviour of my iOS app:
When I start the app for the first time (after some time, like an hour or so), the screen stays white.
As soon as I go to the settings & just press save, I get to my desired sitemap and can use the iOS app without any further troubles.
After two hours of research, I can tell you, that this behaviour only appears when using HTTPS connection (port 8443). When I establish connection via HTTP (8080), everything works immediately without any blank screen.
Finally solved by myself by using NGIX authentication instead of direcly exposing OH2 to the internet.
I can everyone recomment to use openHABian - it takes a lot of worries from your shoulders, especially if Linux is not your hometown