I don't know the internals of KDE so I may be way off here, but I have a hunch that this is caused due to kbuildsycoca5 being missing in Debian sid. I checked the package kdelibs-bin and it doesn't provide an upgrade to v5.
- Bug#790703: kbuildsycoca5 is missing in sid Marcelo Juchem
- Bug#790703: kbuildsycoca5 is missing in sid Maximiliano Curia