I also wanted to confirm that if you are seeing failed login attempts in AD with an account name of a cert that your Veeam backup agent generated and stored in Microsoft's certificate store. Then that is a known issue Veeam Support does offer a hot fix for that you need to get directly from them. If it helps speed up the troubleshooting process with support then reference my case #07202791.
In my case I was managing the backup agents through Veeam B&R and all the failed login attempts would originate from only that server when I checked the AD logs. I would see multiple of the same failed attempts in a single day. The only thing that changed between events was the certificate ID used to login would change depending on what physical server was in that specific backup job running at that time. The physical backup jobs would run with no issues but would just generate the failed login alerts which caused issues for our security team.
Here is one of the certs that were showing up in the logs that was found on one of my physical servers that were backed up by the Veeam Agent:
Here is an example of the AD failed login event that I would get related to that cert:
The login failure event would happen exactly one second before the backup job would run. So I found the Veeam related event that ran at that exact same second in the "Job.VeeamEndpointBackup.log" file:
I believe the highlighted line above was the exact event that generated the failed AD login event.
Support originally thought it might be related "Veeam Backup Enterprise Manager" but I did not have that solution installed.
Hope this is of some help to others out there in the same situation.