This error message relates to a failure to establish communication with the Veeam Agent for Microsoft Windows service. If the Veeam Agent for Microsoft Windows service is confirmed as running, there may be a firewall issue.
This article specifically documents a situation in which the Veeam Agent for Microsoft Windows service cannot start, or gets stuck starting, due to a port conflict.
The Veeam Agent for Microsoft Windows service default is port 6183. As the service starts it will check if that port is in use, and if in use the service will increment and check again (6184,6185, etc.). It will attempt to increment up to 10 times before failing. (Logging example available below)
A much rarer scenario is one in which the port Veeam Agent for Microsoft Windows service attempts to start on is registered in RPC Endpoint mapper by another process but is not actively in use. The service will not detect the port as in use and attempt to start, it will fail to start and will not increment the port it attempts to start on.