After Upgrade to Veeam Backup & Replication 12.1, Backup of Linux VM Fails With: "Failed to check fingerprint"

During the upgrade to Veeam Backup & Replication 12.1, the format used to store SSH fingerprints for Linux machines is updated. However, for environments where the configuration database was stored in a Microsoft SQL instance running SQL 2022 or 2019, the conversion process fails to correctly process blank current_fingerprint values in the Backup.Model.HostSshFingerprint table. Those blank values are replaced with NULL, which is unexpected by Veeam Backup & Replication.

To resolve this issue, manual fingerprint validation must be performed for each machine that failed with the error.

1. In the Main Menu (≡), click Options.
2. Switch to the Security tab.
3. In the section labeled "Linux hosts authentication," change the setting to:
   Add unknown hosts to the list manually (more secure)
4. Rerun the job(s) that failed.
5. Validate SSH Fingerprints and Trust entries within the Untrusted node in the Inventory view.
6. Run the jobs that failed once more.
7. After all jobs have returned to an operational state, you may return the Linux host authentication setting to Add all discovered hosts to the list automatically.