I have kde neon. I added a new user via settings.
When I log in, I get a blank desktop. I can not add panels, it silently fails.
Via run command I can start konsole. From there, I can launch firefox, but I can not launch systemsettings5.
I don't see any error messages in konsole either.

I have removed the /var/tmp caches
I have removed plasma settings. I have restarted plasma.
Nothing I have tried has worked.

There are no system errors than I can see. There is no diskspace problem. My existing user session works fine.

Ah. This was the problem. I have a laptop and two external displays. The laptop is closed, and the laptop display should be disabled.
But in the new session, it was "enabled" (but not visible) and it was the primary display. So all my add panel requests and so on were adding things to a display that was not visible.
I was able to start system settings by adding the system settings widget to the desktop I could see. From there, I was able to fix the monitor configuration.
I will file a bug.