After upgrading to Veeam Backup & Replication 12.1, Backup jobs for Linux VMs that operated successfully before the upgrade now fail with:
Unable to connect to guest OS for guess process. 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.