Is this machine or backup job new, or has it always worked till just now?

If new, I'd first check the credentials work to connect to the VM. In the guest processing section of the Backup job you can do a test of the credentials used. 

Here's a Veeam KB I found which may be of benefit. 

I want to know what the root cause is. The system will retry again and return to normal the backup image. Is there any way to prevent it?

@coolsport00 This Hyper-V guest started doing backup on November 27, 2023, until the problem occurred yesterday. Is it not a configuration problem, but a component problem in the Veeam backup server? Is there any chance of finding a root cause?

Did you take a look at the logs to see what they might say during the failure?  C:\ProgramData\Veeam\Backup

It would be hard to say since Veeam retries and works so to me that would be the important piece here that the job is successful.

@Chris.Childerhose Thank you for your replies. I will try to find answers in the log file. If there are any questions in the log file, I will ask for your opinions. Thank you again for your help.

@Kwok Eddie - no...the cause is more than likely not a Veeam component because other VMs in the job didn't have the issue, but rather only 1 VM. That leads me to believe the issue is happening within the guest OS. You can peruse through the logs Chris shared, as well as the Event logs within Windows of the problem VM to see if anything stands out. 

Hi @Kwok Eddie - were you able to view logs? Did you see anything more descriptive of what could possibly be causing your VM guest processing error?

Hey @Kwok Eddie - do you have an update on this? Are you still having backup issues? Anything further we can help you with?