FOUND THE PROBLEM.
I am using FreeSSL - a service from my hoster “Hostpoint” and forcing all connections to go via HTTPS. Deactivating this option and using http in iOS App works. Read somewhere, that iOS devices (Apple?) do not allow certain certificates. Support Guy from Hostpoint told me, that the FreeSSL certificate doesn’t contain much (just the domain and one other information). Could imagine, that Apple, again, does not allow this. Any confirmed answers to this? Thanks, Daniel