Cannot proceed - another task needs to finish using the Storage Vault first.
Locked by this device (PID #XXXXX) since XXXX-XX-XX XX:XX:XX (XmXs ago)
The other task is still actively running: must wait until it completes.
The retention encountered a problem (exit status 1)
Appears because another backup job has not completed on another device sharing the same username.
The message is about the retention pass which checks old backup versions, and is not related to the backup of your data.
The status of your backup job is listed at the top of the log, and if the backup is ok will show "Backup job saved", therefore it's safe to ignore this message.