Unknown rpc error: code = 3, category = "MDM IPC Error"

Symptoms

MDM link can't be configured, error Unknown rpc error: code = 3, category = "MDM IPC Error" is thrown:

pmm_dep_service.log:
02-10 10:52:18.830 D /Certs:4328:15e8/ Found 1 local certificate(s)
02-10 10:52:18.836 D /MdmIpc:4328:15e8/ Connecting to mdm.xxx.yyy:8761
02-10 10:52:18.837 D /MdmIpc:4328:15e8/ Sending request 'type=EnrollmentUrl id={d9f0ab6e-f2b7-c142-9c8f-09c97a9df346}'...
02-10 10:52:20.849 W /Ipc:4328:15e8/ Connection refused

Cause

The service port of the MDM service is blocked by the firewall.
To verify it, you may use the PowerShell commandlet Test-NetConnection on the server running PMM Proxy.
It must return True when executed against the public URL of the MDM server and its service port:

Test-NetConnection mdm.xxx.yyy -Port 8761

If it returns False, the port is blocked by the firewall.

Resolution

Please adjust firewall settings to allow TCP traffic from PMM Proxy to the MDM server over the service port.