i decided to reinsert the relative path in the 61-openmediavault-dev-disk-by-id.rules (removed with the new version)
and now my serial number show correctly and no more error 'failed with exit code 1'.
I'm wondering because systemd developers told me that it is not necessary to use absolute paths. And it seems no a generic issue, otherwise the forum and bug tracker would be flooded with reports about that. So i assume something else must be causing the issue? Maybe a custom kernel, other udev/systemd packages from sources other than the Debian repositories, ...