The problem seems to be caused by an incompatible version of the Qt libraries:
$ /usr/lib/kscreenlocker_greet --testing
Fontconfig error: "/etc/fonts/local.conf", line 1: not well-formed (invalid token)
kf.kirigami: Failed to find a Kirigami platform plugin
Cannot mix incompatible Qt library (5.15.8) with this library (5.15.9)
fish: Job 1, '/usr/lib/kscreenlocker_greet --…' terminated by signal SIGABRT (Abort)