I tried to manually do a Borg backup, but it failed (some details omitted):

/etc/borgmatic.d/sync.yaml: An error occurred
borgbase: Error running actions for repository
Failed to create/acquire the lock /srv/repos/<name>/repo/lock.exclusive (timeout).
Command 'borg create --info <repo>::{hostname}-{now:%Y-%m-%dT%H:%M:%S.%f} /etc/borgmatic.d/immich.yaml /etc/borgmatic.d/sync.yaml /mnt/data/sync /run/user/0/./borgmatic' returned non-zero exit status 73.

Need some help? https://torsion.org/borgmatic/#issues

As it turns out, Borg maintains some kind of lock in the filesystem and I must have interrupted a backup in progress at some point, so the lock was never removed. The fix is to ensure that nothing is currently doing a backup and then remove the lock using borg break-lock <repo>.

See: https://github.com/borgbackup/borg/issues/3191#issuecomment-338160680