You would have to examine the details of the backup in History - select the plan and make sure the File button on the toolbar is selected. See what is being reported. If a file was in use and locked on the NAS, it likely would not be backed up as there's no support for VSS on NAS devices.

No, I don't use VSS and if I see in the History,after each file I read in the "Message column" "username or password is incorrect".

Remember: The Credential-Test works without error and the username/password is still the same.

Are all files showing the credential error or just a subset of them?
Check two things:
1. Click the Network Credentials in the Tools toolbar and ensure your credentials are entered and they work correctly (it should be the same dialog you see in the backup wizard, but it's worth a check)
2. If that does not address the issue, then maybe the service account is preventing network access. Consider changing the account as needed: https://help.msp360.com/cloudberry-backup/backup/about-backups/backup-service-account

Also, can you confirm the NAS is added as a network share and not a mapped drive?

I don't know if this is related, but yesterday I upgraded to the latest release and I could no longer backup or restore from my Windows share on my Synology NAS. I found an older version (6.3.0.276), installed it and my backups and restores started working again. The error I got with the new version that went away when I installed the older version was: “a specified logon session does not exist. it may already have been terminated”