Oh well, updating the rm
reference seems to have fixed it.
[Service]
ExecStartPre=+rm -f /var/lock/LCK..ttyAMA0 /var/lock/LCK..ttyACM0
Issue was introduced in early August 2023 in this commit: Remove serial lockfiles before start · openhab/openhabian@e0250ae (github.com)
where it is assumed that /usr/bin/rm
exists.
UPDATE: Better solution is to copy the file from /bin
to /usr/bin
so updates will work in the future too.