First of all, make sure Veeam ONE Monitor Server service is up and running.
In case the service is running properly, confirm the following:
1. The name of the Veeam ONE Monitor Server is correctly specified in Veeam ONE Monitor client settings.
2. The Veeam ONE Monitor Server is reachable from the machine where Veeam ONE Monitor Client runs.
3. In multi-Monitor Client environment, connection to Veeam ONE Monitor Server may fail with one of the following errors:
Service connection failed Address = \\IP_Address\pipe\VeeaMDCS Error = 0xe7
Service connection failed Address = \\IP_Address\pipe\VeeaMDCS Error = 0x5
To resolve this issue, add the Veeam ONE Monitor Server service account to the Veeam ONE Administrators security group.
4. The ports used by Veeam ONE Monitor Client to connect to the Server have to be opened:
Besides these standard ports, Veeam ONE Client also uses dynamic TCP ports 49152 - 65535. Check if the mentioned ports range is open on the firewall.
NOTE: For versions 6.x-7.x of Veeam ONE, the account that is used to run Veeam ONE Monitor Client must be included in the Veeam ONE Users security group on the Veeam ONE Server.