The Alt-Tab switcher does not show running applications and cannot be used to switch windows. It only shows the "Show Desktop" icon and shows Desktop on pressing Alt-Tab. Pressing Alt-Tab shows the task switcher ribbon but shows no running applications.

Having this same problem, so bumping in hopes someone knows a solution.

I don't know if vdhande ever found a less drastic solution, but I ended up nuking my ~/.config/kwinrc, and re-setting up my window appearance settings.

I tried deleting several other KDE-related files in ~/.config before this, I'm not sure how I missed kwinrc.