Error when attempting full site backup

Automate emails “Error when attempting full site backup”

Managed Backup scheduled weekly, but the last backup date is on March 13. Tried once off backup, but it fails first showing General backup error. The log shows

[snapshot][2018-04-05 07:21:42][Error] Error processing manual backup: unknown:unknown

[snapshot][2018-04-05 07:21:53][Error] Error processing manual backup: backup

Then when tried again the managed backup run. Any suggestions.

Regular backup works well.