black desktop in KDE on primary monitor

I have Kubuntu 20.04 LTS version. I have two monitors connected to my PC - primary one via HDMI and secondary via DisplayPort. Both have the same resolution. After login into KDE I have the 'task panel' and the 'application launcher' on the correct (primary) monitor. But sometimes I dont have any 'desktop' - just black screen (when I minize all windows) on the primary monitor. I have correct desktop wallpaper on the secondary monitor - but of course, without any widgets since they are placed on the primary one. I don't know exact steps to reproduce, because it seems this is happening randomly - sometimes I have the desktop wallpaper and sometimes i don't. In all cases I have wallpaper on secondary monitor and I have the 'main panel' and 'application launcer' visible on the primary one (but not the other widgets, icons and wallpaper) This happens on different activities too - for some I have the desktop layout as Desktop and for other I have it as Folder View Any idea what can couse this and/or how to fix it?

Thanks

I have to use the following command from terminal:

kquitapp5 plasmashell && kstart5 plasmashell &

but yes I am also looking for solution to make it start automatically as in single display.

I have had the same problem. Kubuntu 20.04 LTS, Primary monitor HDMI-1, Secondary VGA-1. In System Settings...Hardware, Display and Monitor...Display Configuration I found disabling/enabling and switching primary display eventually brought the desktop back but the taskbar/panel ended up on the secondary monitor. Right clicking the primary desktop...Add panel...default panel brought installed a new task bar on the primary monitor leaving a duplicate on the second. Right clicking on the secondary panel...Edit Panel...Remove panel...X to close edit returned everything to normality.

This is a workaround, not a fix.