SUMMARY

I have two monitors, a landscape 1680x1050 (main monitor) and a portrait 1920x1080, rotated 90 degrees anticlockwise (secondary monitor). Nvidia graphics card, with the proprietary drivers.

When I log in, the secondary monitor is entirely black. If I move my mouse to the monitor, I can see it. If I try to drag a window to it, I can see the mouse cursor but no window.

If I use Ctrl-Alt-F2 to swap to a console, then swap back to the desktop the problem is resolved and I have a notification stating that kwin restarted desktop effects due to a graphics change.

STEPS TO REPRODUCE
1. Starting point - everything is displaying as expected, with one monitor landscape and the secondary monitor portrait
2. Open System Settings/Display Configuration
3. Disable the secondary monitor and click Apply
4. Enable the secondary monitor (this will have reverted to landscape mode) and click Apply
5. -> The monitor should now display the desktop as normal (although in the wrong orientation in my case)
6. Click on "90 degrees anticlockwise" orientation, then click Apply
7. -> The monitor will be black, with a visible mouse cursor

The same behaviour is shown going from no rotation to 90 degrees clockwise, or from 180 degrees rotation to either of the portrait orientations, but it works fine going from no rotation to 180 degrees.

OBSERVED RESULT

The secondary monitor is entirely black, but with a visible mouse pointer. Attempting to move windows to the secondary monitor shows the mouse pointer but no window.

EXPECTED RESULT

A normal desktop showing on the monitor.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 20.04
KDE Plasma Version:  5.18.4
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

kwin version: 5.18.4.1-0ubuntu2

ADDITIONAL INFORMATION

This is easily reproduceable, I can provide more details / debugging if needed.