I use rdiff ever since, for some years via backupninja. Some time ago, a
backup failed, and I started to get this error message:
== fatal errors from /etc/backup.d/90.rdiff ==
Info: Removing backups older than 400D days succeeded.
Error: Previous backup seems to have failed, regressing destination now. Warning, could not find mirror_metadata file. Metadata will be read from filesystem instead. Fatal Error: No metadata for time Fri Dec 2 20:06:38 2016 (1480705598) found, cannot regress
Fatal: Failed backup up source