Ah, snap. Indeed, I intentionally stopped the apparmor.service to get LighDM’s guest session working. See Enabling Guest Session for LightDM
Now that I started AppArmor back again, all the snaps (including) Budgie’s Welcome App are working properly. But now I have a XOR situation I can’t get both (guest session and snap) working at the same time for snap works with and guest session works without AppArmor, only.