I have a very strange problem with using an external monitor connected to my Dell Latitude 3550 Laptop/

I have installed Linux Mint 18 with KDE5 a few weeks ago. I intalled nVidia and Intel proprietrary drivers, lowlatency kernel and KX Studio repositories with audio software I use.
About a week after that one day I resumed the laptop from being suspended and my both screens were off and I couldn't get them to light up.
There weren't just black. They were OFF - like disabled by power management. No backlight.

I discovered that if I disconnect the external monitor the laptop display goes back on.

By running the Display Configuration program, closing it and rerunning it a few times I can somehow get both screens to work.

This video shows in detail what is happening:
https://www.youtube.com/watch?v=k1uIBEvUz9M

Have you had similar problems? Could this be a bug in the Display Configuration KDE program?