Troubleshooting Tip: FortiGate to FortiClient EMS connection error: 'Connection failed with unknown issue'

Below is the error message on the GUI:

Error On CLI:

FortiGate-LAB# diagnose endpoint fctems test-connectivity 2
Connection test had an error -101: EMS server connection failed with unknown issue. curlcode: 56

Connectivity is established as Telnet connects with the specific port number.

Collect the below debug on FortiGate CLI:

diagnose debug app fcnacd -1 

diagnose debug enable

Replicate the issue and disable the debug by below commands :

diagnose debug disable

Debug output :

[ec_ez_worker_process:400] Processing call for obj-id: 0, entry: "api/v1/system/serial_number"
[_update_obj_stats:365] Storing (0, EMS,
[ec_ez_worker_process:458] Call completed with failure.
obj-id: 0, desc: "REST API to get EMS Serial Number.", entry: "api/v1/system/serial_number".
error info: Error (__generic_process_result_ex:169). CURL error: (56)Failure when receiving data from the peer.  <----- FortiClient EMS is rejecting the connection.

Solution:
This is due to the compatibility issue between Windows Server and FortiClient EMS firmware version.

Check the system requirements for FortiClient EMS version from the FortiClient EMS Administration Guide and upgrade accordingly: System requirements