After the system went into standby, when I unlocked the screen one of 2 monitors (the primary one at 4K) is black, no mouse pointer, nothing! reboot doesn’t help. In display settings the monitor is present, but not activated. I find and select it from the drop down list, and can change the resolution, but the refresh rate is not selectable, and it reads 30Hz with an i next to it, I click the i and get "30 Hz is the only resolution this monitor can display (it can do 60Hz for sure). I log off and log on to a different user account and all is well, both monitors work, and as usual.
Conclusion: Something wrong with Plasma settings of that one user account only.
Now, for the last week or so (I run updates daily) Plasma has not remembered any window sizes or positions, nor Icon locations after moving on the desktop between reboots. They seem to be stuck in time from when I used them one week ago.
Since only one user account is affected it must be related to one or more user specific Plasma display settings related files, and Plasma is not making changes to it/them, but now somehow did, but in a very bad way! Maybe it’s a module it uses to record changes, but I cannot tell yet until I logon to the other user account open a few apps resize their windows, move a few Icons around… reboot and see if the changes took, but I don’t want it to break too!