Created attachment 124431 [details]
Screenshot with message in red.

SUMMARY
After starting my PC this morning, the virtual desktop pager in the panel no longer shows any applications on my secondary monitor.  Thinking it may only be missing the thumbnail of the app, I tried to grab the space it would be in and drag it to another desktop, but that doesn't work.  The only way I can move apps between desktops for that monitor is by closing the app, switching desktops and reopening it. I can move apps from my primary monitor to the secondary one via the pager, but as soon as I do, it doesn't show up in the pager, and I can no longer move it to another desktop, only to the other monitor on the open desktop.

STEPS TO REPRODUCE
1. Open any app. 
2. Place on the secondary monitor.
3. See results in the pager.

OBSERVED RESULT
App not shown in the pager.

EXPECTED RESULT
App shown in the pager, and able to drag to another desktop.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 
Qt Version: 5.13.2-3 (base)

ADDITIONAL INFORMATION
I install updates daily, and did last night before shutting down, but there were so many I don't remember if there was one that could have caused this.

If you need any system file, just ask, and please give me it's path location so I can find it.