Had the same problem. Keyboard stopped working on a single user, and only on KDE, so had to be a local issue. Thanks for solution.
Thanks for the tip. Turning off slow key solved my problem.
I see now it's a "feature" of KDE accessibility tool. I probably activated it by pressing the shift key for 8 seconds. This is the mesage I got after turning it off:
Warning - KDE Accessibility Tool
Do you really want to activate "Slow Keys"?
You held down the shift key for 8 seconds ...
There is an option to "Deactivate All AccessX Features & Gestures."
I also see an option to disable AccessX features in:
Control Center -> Regional & Accessibility -> Accessibility
Uncheck Activation Gestures -> Use gestures for activating sticky keys and slow keys.