1) Open the Veeam B & R Console.
2) Select Backup Infrastructure.
3) Select Managed Servers.
4) Select the vCenter Server that is used to backup your Cluster/Farm.
5) In the "Edit VMware Server" screen click on credentials. (You should not have to change anything
here).
6) Click next.
7) You will get an error about the certificate and be asked if you want to continue anyway.
8) Continue the process and let it go through the "Detecting Server Type" and save the server
configuration.
9) Go to your Backup Jobs and under "Virtual Machines to Backup" select each entry and click remove. Make a note of any exclusions etc. and add them back in exactly as they were before.
10) After you have added them back in go down and click "Recalculate".
You should see the size of the backup is calculated correctly. It may have said "N/A" before.
11) Re-run the backup job by selecting "Retry" or "Start".
This should clear the error and your backup job should run now as before. I hope this helps someone else with the problem. If not, maybe it will put you on the right track.