@fossfreedom It’s been a while but now I found time to test your AppArmor suggestion.
I stopped the AppArmor service and disabled it. First it seemed that these steps didn’t change anything. But then I additionally restarted my computer while keeping the AppArmor service disabled and now my guest sessions works 
Horray!
With some more testing (enabling and disabling AppArmor) I can confirm, restarting the computer after disabling AppArmor is indeed necessary. I hope this will help others.