What version of Veeam 12 are you running?  What operation are you trying to run when the error happens.  We need more details otherwise we are just guessing.

Please add as much detail to get some guidance as we are not technical support but can help as best we can.

error occurs when I run backup jobs on hyper v virtualized servers

Hi @DuongIT 

as Chris wrote we are not official support, and anyway they'd ask you the logs. 

To reply to your issue we need to have more and more info. 

Probably your Hyper-V host is not detected but we cannot doing any troubleshooting. 

Let us know how we can help you.

Can you reach the servers HyperV properly ?
Do you have right config DNS ?
@DuongIT You need to be more detailed in your infrastructure information….

I can still access normally, the error only happened yesterday, before that the backup jobs were still normal

I think you have already checked to see if any Windows Updates have been made on the disputed servers... including DNS configured on the Veeam server's nic.

Is there any way for tasks to run normally?

are your Hyper-V servers configured in a failover cluster?

Its possible they have failed over but the backup job is configured via the host rather than the cluster.

Is there any way for tasks to run normally?

Yes when a job runs it is normal. Based on your issue you are better off opening a support ticket and working with them. We still need a lot more information on your environment and support can use the logs to get to an answer faster for you.

What version of Hyper-V are you using?  Is the server fully patched?  Is DNS working correctly?  Are the credentials being used correct and no expired password?

@DuongIT -

As Chris shared...we’re needing a bit more info to help you here.

1. Is there error you’re getting related to a “managed server” (i.e. a Veeam component such as Repository, Proxy, or other)? Or, is the error related to a VM you’re trying to backup?
2. Do you have any Veeam Components → Hyper-V, Repository, Proxy, etc which show as ‘unavailable’ in the Inventory node?
3. You can also check the Backup logs in: C:\ProgramData\Veeam\Backup\<job-name>.log to see if there is more detailed info there. 
4. Can you share any screenshots?
5. At the very least, as Chris shared, opening a Support ticket with Veeam will probably serve you best.

Any port changes as of recent or FW updates?