Here is the ticket #00140594

Is what you posted the exact error you are seeing? If not, please post the exact error
Yes, it is the error exact in logs : " ERROR - CBL3018: The backup storage server returned an unexpected response"
Where are you backing up? Any changes to your storage (if this is a local backup)?
The backup destination is S3 and any changes to the storage.
Are you using the new backup format or the older format?
I use the older backup format
Is this a new backup, or were you running it successfully on an older version of the agent, and then you upgraded and started seeing this error?
The backup is successful with the previous version (7.3.2.6) but after upgrading the version to 7.4.0.161, some backup plan failed.

I have a reply from support. It seems that the problem is related to the errors of VSS. On both machines, the service account did not have the necessary rights to take a snapshot. We changed the account to that of the Local System and the backups are working. Thank you !!!