run-parts was returning an error code (2 IIRC) because I had removed, but not purged, update-manager-core from these systems a few days ago.
That resulted in /etc/update-motd.d/91-release-upgrade being left behind, which would attempt to run /usr/lib/update-manager/release-upgrade-motd which no longer existed.
After purging update-manager-core, /etc/update-motd.d/91-release-upgrade was also removed, run-parts now completes successfully and the MOTD is displayed correctly.
I "fixed" this before seeing comment #2:
run-parts was returning an error code (2 IIRC) because I had removed, but not purged, update-manager-core from these systems a few days ago.
That resulted in /etc/update- motd.d/ 91-release- upgrade being left behind, which would attempt to run /usr/lib/ update- manager/ release- upgrade- motd which no longer existed.
After purging update- manager- core, /etc/update- motd.d/ 91-release- upgrade was also removed, run-parts now completes successfully and the MOTD is displayed correctly.