Hi Jeroen, if you are using Testing, you have an incomplete Plasma installation. When you upgrade Plasma packages, be sure to always upgrade the entire stack at once.
We've uploaded a fix which should prevent this issue in the future. In the meantime, you can downgrade kdecoration to 5.21.5 by grabbing the debs from here: https://snapshot.debian.org/package/kdecoration/ 4%3A5.21.5-2/ -- Med vänliga hälsningar Patrick Franz
