This is what i found when searching for the error:

I Guess its worth a try to check. DNS is a common issue when trying to connect to shortnames of Servers from a non domain joined VBR Server. 

This Veeam error indicates a problem with network communication, most commonly caused by a firewall blocking ports, an incorrect hostname or IP address, or the Veeam service not running on the target server. To troubleshoot, check the specific port Veeam is using (like port 10005 for agents connecting to a backup server) and confirm network connectivity between the client and server using tools like ping or nslookup. 

Troubleshooting steps

1. Verify network and hostnames 

• Check connectivity: Use ping from the machine running the Veeam client to the server in question.
• Resolve hostnames: Use nslookup on the client to ensure the server's hostname, FQDN, and IP are resolving correctly. If not, a firewall or DNS issue is likely the cause.
• Use the correct IP: Confirm that the server's IP address is correct in the job settings and that the correct network interface is being used, especially on servers with multiple NICs. 

2. Check firewalls 

• Local and network firewalls: Verify that firewalls on the client, the server, and any network firewalls in between are not blocking the necessary Veeam ports.
• Specific ports:
  • For agents connecting to a backup server, the default port is 10005.
  • For Veeam ONE, check the server to ensure the Veeam ONE Monitoring Service can communicate without issues.
• Firewall features: Disable or exclude traffic from features like "signature-based scanning" or "SSL inspection," as these can interfere with encrypted Veeam traffic. 

3. Confirm Veeam services are running 

• Check services: On the target server, ensure that the required Veeam services are running.
• Restart services: If the services are not running or you suspect an issue, restart them. You can do this from the Veeam Help Center or directly on the server. 

4. Look for other potential causes

• Port conflicts: Another application might be using the same port. Check if the backup service can assign itself a different port.
• Windows Updates: A recent Microsoft security update can sometimes interfere with communication, especially in Hyper-V environments.
• Permissions: For certain jobs (like VMware), ensure the account specified has the necessary permissions.
• UAC: On Windows, User Account Control (UAC) can sometimes interfere with communication. 

TCP Ports from 2500 to 3300 are only used on demand. It only listens when transport agent is started (Veeam.Agent name in the process view) and then first connection use TCP2500 and next one 2501, next one 2502 and so on.

From my point of view is a Firewall issue.