Identify which proxies the Job was allowed to use, then review each of those proxies to identify which transport mode they were configured to use. Then, review that transport mode's requirements and ensure they are met for the VM that failed to be backed up.
Step 1: Determine which Proxies the Job was permitted to use.
Step 2: Check for Proxies that are Unavailable or Disabled
Having identified which proxies could have been used by the Job, review each proxy and ensure that they are not listed as Unavailable or Disabled.
If any VMware Backup Proxy is listed as Unavailable, identify its related entry under Managed Servers and rescan it. If the entry under Managed Servers fails to rescan, edit it instead and "Next, Next, Finish" through each of the pages of the wizard to force Veeam Backup & Replication to check if services on that remote machine need to be redeployed.
Step 3: Check if Transport Mode restrictions are in place
Review the Transport Mode selection to identify if any of those proxies have been configured to only use a specific Transport Mode.
Note: The Transport Mode selector is set to Automatic in most environments to allow the Veeam Backup & Replication software to detect the most appropriate Transport Mode to be used for each VM. The option for "Failover to network mode" is left enabled to ensure that even if Direct storage access or Virtual appliance (HOTADD) fails, the Proxy can still utilize network transport mode as a last resort.