Thank you for the Links. I couldn't find any new information that I did not already checked and confirmed in my post though. Here are the screenshots for the working purchase flow when I test it on my device when I run it locally. Hmmm…

And this is the flow in Testflight starting directly from the settingsview:

Never mind and thank you! I need to react since I can't see that red errors one more day. I'm going to delete the IAP from the App and Appstoreconnect for now and take care of it later. Thanks again