Unfortunately not the answer :-) As said before, my App is for a Mac, not a phone. Of course it can run on the development machine .. but giving it to others, need signing and notarization. Apples documentation is unfortunately not precise. They say, you need a paid account to distribute. No info on the question, if "distributing" means the app store for Mac only, or also the distribution of a pkg or imaga or even only a zip with the app folders inside.

Thank you. That's the information, that I was looking for.

In the meantime, I found out that the problem is caused by codesign. but still no idea, why ..

The problem is caused by implementing the entitlements file. It contains only the sandbox line:

com.apple.security.app-sandbox

I found the reason in the machine's Console: There was an issue with permissions. I had to add network permissions and then the app was running. BUT: The installer (pkg) quits with a "sucessfully installed" - but the app does not appear in the "Applications" directory. Could someone help me out with the reason? I think, it's again a problem of Apple's "Gatekeeper" stuff ..