HDMI monitor connect results in desktop logout and start of new session

I have a strange problem with HDMI.
If I connect my HDMI external monitor to my laptop I will get a new login screen. It takes quite a long time to login than.
When I switch off the laptop it takes about 3 minutes to switch off. While it switches off I see that I have to wait because there is another session actief.

I worked since the start of 15.4 with the laptop and never had that issue before. It only appeared a few weeks ago.

If I start the laptop with HDMI already connected I get one login but the same when I shut the system down.

I have a laptop with an nvidia dedicated GPU

No one with a reply.
I actually could revert with snapper to about 2 month ago and that fixed the problem.

If this problem returns, instead of logging in on the new login screen, switch to the other vttys to see if your original post-login environment screen appears on one of them. Ctrl-Alt-F[1,2,7,8] will probably produce your original. On the others I expect would either be some boot messages on F1, or shell login prompts on all the others.