Have you tried either a factory reset or clearing the cache in recovery? I am not sure how to do either, but it is typically a good thing to do after system updates.
I don't feel like setting up the phone again. It works flawlessly except of the shutting down issue. For the moment I think I prefer waiting for an update that might fix it, at least until I get fed up of checking my phone frequently for whether it's still running or not.
As I said in my initial post, I started the thread to see if there are no other users in this forum with the same issue, and to let off some steam.