seems to be a unity problem!
Solution I found:
Games that use Unity do not work and give an error:
IndexOutOfRangeException: Array index is out of range.
at UnityEngine.Display.RecreateDisplayList (System.IntPtr nativeDisplay) [0x00000] in <filename unknown>:0
I went into Nvidia Control Panel. This is where I changed "Aspect ratio" to "No scaling" on the Adjust desktop size and position page (with "perform scaling on Display"). Suddenly everything was okay again. In DS3 I had options for resolutions. If I started up a Unity game with shift, the Select Monitor was no longer empty, but said "Display 1".
I tried switching back to "Aspect ratio" to see if this would break things again. It did for some games (e.g. Hearthstone), but others kept working. I also tried "Override the scaling mode set by games and programs" and that seem to work as well.