Capacitor app cannot load files via JS in iOS <13

We built a Capacitor app, which runs on iOS 14 and 13, but runs into problems on iOS 11 and 12.
Capacitor is started and the index.html is loaded, so is our Javascript code. But the JS code itself cannot load files (the App starts by reading an XML file with the app content and structure).
It looks to me like a permission issue (JS not allowed to read files) or a problem with URL handling.
I am thinking that probably the AppDelegate has to be modified in some way?

Maybe someone had similar problems and can give me a heads-up.