3518358 - OAuth Client connection test failed with NIECONN_REFUSED(-10) error

The following error message is raised when running the OA2C_GENERIC_ACCESS report to test the OAuth Client connection: 

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HTTP failure, processing failed, invalid state, invalid timeout or others
oa2c_error
Client configuration error or network problems. See kernel traces.
HTTPC Method:                  RECEIVE
HTTPC Subrc:                       1
HTTPC Subrc Text:              http_communication_failure
HTTPC get_last_error code:       411
HTTPC get_last_error message:
Direct connect to <configured token endpoint hostname>:443 failed: NIECONN_REFUSED(-10)
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NIECONN_REFUSED(-10), proxy, OA2C_CONFIG, OA2C_GENERIC_ACCESS, OAuth Client, http_communication_failure, oa2c_error, 443 failed: NIECONN_REFUSED(-10), 411, 417 , KBA , BC-SEC-LGN-OA2 , OAuth 2.0 for ABAP , Problem