I finally found the solution. And it is actually very simple.
In your Engine install directory you have to open BaseEngine.ini
In there you have to set the following variable to false:
[/Script/MacTargetPlatform.XcodeProjectSettings]
bUseModernXcode=false
This forces the Engine to use the old distribution packaging method.
It was not enough to uncheck it in the project settings. It needs to be set in the base engine.ini file.