Saw this rsnapshot error this morning on a couple new Xen domUs that I created last week:

[08/Mar/2010:08:12:53] /usr/bin/rsnapshot hourly: ERROR: /usr/bin/rsync returned 255 while processing root@foo.bar:/
[08/Mar/2010:08:12:53] touch /backups/hourly.0/
[08/Mar/2010:08:12:53] rm -f /var/run/rsnapshot.pid
[08/Mar/2010:08:12:53] /usr/bin/rsnapshot hourly: ERROR: /usr/bin/rsnapshot hourly: completed, but with some errors

Turns out I hadn't ssh'ed to the domUs from the backup machine though I had copied the public ssh key for backups there. As such there wasn't a local stored ssh fingerprint of the domUs and rsnapshot was failing. Easy fix, just ssh foo.bar once from the backup host and accept the fingerprint :)