Okay, I was able to run the game in standalone mode (PIE) and complete the packaging process now and all under 21.2.
Sadly I didn’t really figure out what the actual problem was. After fixing most of the errors and warnings I recognized most of the errors left were linked to a marketplace plugin for procedural foliage creation. I’ve stopped using this plugin in this project but there were still some references left in some blueprints so I couldn’t delete the folder yet. However I did delete it now, fixed all the broken references and Then everything worked.
The plugin itself should work perfectly fine under 21.2 so I don’t quite understand why this was an issue but at least it is working now.