I had a Veeam Agent backup running regularly and successfully. The agent server is a simple Windows Server 2016 file server. One day recently, the backup job simply stopped working.
When I manually ran the job, it started OK, it got as far as “Required backup infrastructure resources have been assigned” being green-ticked, and then nothing. No further progress, until around 30 minutes later the job failed with “Error: Managed sessions <session ID string> has failed.”.
All other backups, Windows and Linux, were working fine.
I tried the following:
- Restart the Veeam services on the agent server
- Reboot the agent server
- Run Windows Updates and verified the Veeam agent was up to date
- Checked the firewalls
- Verified the DNS A records of both servers, including reverse DNS – this is something that came up on some googling I did
- Ping the servers from the other one, in both directions
Nothing was obviously wrong, but nothing worked – those jobs kept failing in exactly the same way.
I did notice one thing though – the ping from the agent server to the Veeam server used IPv6, even though it isn’t something we use on our network.
Most importantly, we don’t have IPv6 DNS. Maybe the agent server was mistakenly using IPv6 to find the Veeam server?
Certainly the agent server didn’t need IPv6. So, I tried switching off IPv6 on the agent server – just by unticking it in the Ethernet properties, and then running ipconfig /registerDNS.
Success – I then retried the backup and it went through straight away.
Heavy Soil 
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