Is the Veeam Backup (or Proxy) server in the same subnet as vCenter Server/ESXi?
Are there any network firewalls active between subnets?
Can you confirm that the Backup Server has proper network access to vCenter/ESXi, i.e. is not redirected by a Proxy server setting on the Backup Server?
See also whether https://www.veeam.com/kb1198 helps.
The Veeam Backup Server is in the same subnet as vCenter Server/ESXi.
Veeam Backup Server to vCenter Server/ESXi network is OK. I can access them on the Veeam Backup Server.
This is a network issue. It has to do with distributed switches.
I change to other network, then it's OK.