Thank you so much. Looks like I have to contact the provider of the hosted UI as the issue seems to be on their side then.

Thanks, I did not know you could test your links via the developer settings. Turns out my universal link works perfectly fine. I was thinking so because when I open a hosted UI in my app that redirects back to my link, the web view goes to the remonon.com webpage instead of back to the app. Do you know if it’s possible to link back to your app this way? I was told by the provider of that hosted UI that this would be possible.

Please ignore that my username is "Deutsche Post DHL Berlin". I am currently in contact with apple developer support because I cannot change it.