Make sure that the CIM Server service is running on all ESXi hosts that are affected. This can be done be done from Configuration > Security Profile > Services ( Properties )
with the host selected in the vSphere Client. In the services properties window, select Options with CIM Server selected in order to start the service.
Also, as stated above, make sure that the CIM Secure Server
port ( TCP 5989 ) is opened on the host from Configuration > Security Profile > Firewall ( Properties )
with the host selected. The port may also be blocked by an external firewall as well.
The CIM Interaction
privilege is also required for the account connecting the Veeam MP to vCenter as detailed on page 12 of the Installation Guide.