Fortunately I found the fix. Logout, do:
rm .kde4/share/config/plasma-desktoprc
and login again.
It gets better and better.The taskbar now lost it's transparent color. It's now using a bright, distracting color. I tried to move it to another side of the desktop. When doing that, the color looks OK again. When I move it back, it's broken again.
While doing that a few times, my computer started freezing periodically, including the mouse cursor, and the HD trashing. What the hell? Fortunately there's CTRL+Alt+F1. I pressed that, and after 30 seconds it got registered. dmesg shows nothing. htop shows... wow! A process called "kactivitymanagerd" (I think) was using 15.7GB RAM (it's a 16GB system) and 8GB swap (and swap usage was getting higher by the second.)
That's what I call defensive programming, lol. When you have used up all resources available, well fuck that, keep going.
PS:
And I still have no idea how to fix the taskbar/panel color.