I had setup KDE neon user on my laptop, triple boot with windows and Elementary OS. It was working fine with no signs of issues. After a reboot, before which I had done no significant changes to the system, nearly no apps were working.
Firefox, system settings, Konsole, Dolphin,etc none were working.
Fortunately, I had installed Yakuake and could use that to see if any other apps were working. As it turns out gparted and eclipse were working.

I tried renaming ~/.kde and rebooting, It didn't change anything.

Could anyone tell me how to go about solving this problem. It is driving me nuts.

I have the same problem and reported it here:
viewtopic.php?f=309&t=140034
Looks like there is already a bug report made.

There's a slight difference. In my system, there is no error message popping up.
Also, the desktop looks visually fine.

Also, i noticed that sometimes, yakuake works, sometimes not.
And firefox only responds to mouse activity, not keyboard activity.