The API is running on my own little VPS (on an Nginx-server, served on port :8080, so not via HTTPS).

I think that's the problem. ios doesn't allow HTTP calls by default. The best thing is to use HTTPS. If you really don't want there is an option to enable http

https://developer.apple.com/documentation/bundleresources/information-property-list/nsexceptionallowsinsecurehttploads

Hope it helps someone who will pass by.