Error: The RPC server is unavailable

[PLN]

I'm using the Disk Space test.
If I have a VPN tunnel between Hostmon server and a Passive Client then no problem.
If I change to the public IP address then I get the above error.
The Client tells me I've successfully connected.
I've enabled all the RPC services on both machines, I can ping the public IP address form the server.

[KS-Soft Europe]

I'm using the Disk Space test.

Do you mean Drive Free Space test ?
What HostMonitor version do you use ?

If I change to the public IP address then I get the above error.

If this change is related to HostMonitor <> RMA agent connection, then it should not lead to problems,
unless RMA agent checks servers located on network, where HostMonitor is located.
"Error: The RPC server is unavailable" means RMA cant connect to RPC service on remote system. This can be cause by invalid IP/hostname or Firewall/Antivirus blocks ports, used by RPC service on remote system.

[PLN]

yes, I want to monitor the free space on drive C:

[PLN]

The only difference I can see between a connection over a tunnel (with all ports open) and directly to the IP address is the firewall at the remote site. I've checked with telnet and traffice gets forwarded (NAT) to the correct server and there is no blocking of outgoing ports.
Does the RMA use any other ports that 1055?

[KS-Soft Europe]

Passive RMA uses only one TCP port (default is 1055) for communications between Hostmonitor <-> RMA.
RMA agents use standard Windows protocols (with corresponding ports) to perform tests.
Actually "Error: The RPC server is unavailable" means HostMonitor was able to connect to RMA agent, while RMA agent couldn't connect to RPC service of target system.
Could you try to setup additional test items for this target system ?
You may setup simple Ping test, WMI test, Process and CPU usage test