Hah, I was just dealing with this not 10 minutes ago on one of my desktops. Did you search? There’s a thread on it:
First I tried the advice to downgrade powerdevil and ddcutil to the last known working versions. But the renaming of KDE packages has caused the dependency check to fail for the older packages.
Rather than try to force it, I tried out the workaround in this thread, editing the plasma-powerdevil.service file:
https://bbs.archlinux.org/viewtopic.php?id=289979
Immediately after reboot, the problem is gone. I haven’t done any long term testing to see if it stays fixed with this change.
There is a different workaround documented in the KDE bug report for this issue, changing the service’s .desktop file. I haven’t tried that one.