Checking a Zimbra Network Edition server I found out backups were not running for some time. Nobody noticed because reports email were not delivered.
zmbackup -f -a all
I got this error message:
Error occurred: system failure: unable to retrieve latest session-account, account name-id maps
Talking in IRC Tonster suggested that the main Zimbra backup file, accounts.xml, could have been corrupted.
Since backups were not running since rather long time I had nothing to lose. Thus I renamed accounts.xml to accounts.xml-BACKUP (and the same for accounts.xml.old) and backup is now running again.