Getting a step further as the problem is currently situated in the X-headers.

21:52:28.951 -> Connection to radmon monitoring platform Disconnected.
21:52:28.951 -> Connection to uradmonitoring platform succeeded!
21:52:28.951 -> /01/1621540348/02/21.90/04/47.00/0B/60/0C/406.39/10/0x6
21:52:28.951 -> I am here now 01
21:52:31.422 -> X-User-Id:XXXX
21:52:31.422 -> 
21:52:31.422 -> X-User-Hash:YYYY
21:52:31.422 -> 
21:52:31.422 -> <strong>X-Device-id:0x13000212</strong>
21:52:31.422 -> 
21:52:31.422 -> I am here now 02
21:52:34.401 -> I am here now 03
21:52:34.447 -> <strong>Status code: 200</strong>
21:52:34.447 -> <strong>Response: {"error":"Invalid Device ID"}</strong>
21:52:34.447 -> I am here now 04
21:52:34.918 -> I am here now 05
21:52:34.918 -> Connection to uradmonitoring platform Disconnected.

Looks like the connection get established (HTTP code = 200), my X-headers are validated, yet I receive “invalid device ID” for my device.

Tried “%08lX13000212″,”0x13000212” and “13000212”.

My current code:

// URadMonitoring credentials
const char *USER_ID = "XXXX\r\n";
const char *USER_KEY = "YYYY\r\n";
const char *DEVICE_ID = "13000212\r\n";

... minimized code ...

// Upload data to the uradmonitor.com server
void connecToURadMonLogger()
{
  WiFiClient wifi;
  HttpClient client = HttpClient(wifi, "data.uradmonitor.com", 80);

  Serial.println(F("Connection to uradmonitoring platform succeeded!"));

  // Get current time as UNIX time
  time_t epoch = pftime::time(nullptr);

  // Concat data for POST
  String ptr = "/01/";                      // compulsory: local time in seconds
  ptr += epoch;                             // time epoch value
  ptr += "/02/";                            // 02 = optional: temperature in degrees celsius
  ptr += dht.getTemperature();              // temperature value
  ptr += "/04/";                            // 04 = optional: humidity as relative humidity in percentage %
  ptr += dht.getHumidity();                 // humidity value
  ptr += "/0B/";                            // 0B = optional: radiation measured on geiger tube in cpm
  ptr += totalCount_3;                      // a-cpm value
  ptr += "/0C/";                            // 0C = optional: high voltage geiger tube inverter voltage in volts
  ptr += displayTubeVoltage();              // tube voltage value
  ptr += "/10/0x6";                         // 10 = Tube ID | 0x6 = GEIGER_TUBE_SI22G
  
  // Test output
  Serial.println(ptr);

  Serial.println(F("I am here now 01"));

  client.beginRequest();
  client.post("/api/v1/upload/exp");
  client.sendHeader("Content-Type", "application/x-www-form-urlencoded");
  client.sendHeader("Content-Length", ptr.length());
  client.sendHeader("X-User-Id:", String(USER_ID));
  client.sendHeader("X-User-Hash:", String(USER_KEY));
  client.sendHeader("X-Device-id:", String(DEVICE_ID));
  client.beginBody();
  client.print(ptr);
  client.endRequest();

  Serial.println("X-User-Id:" + String(USER_ID));
  Serial.println("X-User-Hash:" + String(USER_KEY));
  Serial.println("X-Device-id:" + String(DEVICE_ID));

  Serial.println(F("I am here now 02"));

  // read the status code and body of the response
  int statusCode = client.responseStatusCode();
  String response = client.responseBody();

  Serial.println(F("I am here now 03"));

  Serial.print("Status code: ");
  Serial.println(statusCode);
  Serial.print("Response: ");
  Serial.println(response);

  Serial.println(F("I am here now 04"));

  uploadOk = true;
  retry = 0;
  // 5 blinks for 50 ms
  blinkLedUpload(5, 50);
  
  //client.flush(); // Flush connection
  //client.stop(); // Stop client
  //delay(5);// Give some time to stop

  Serial.println(F("I am here now 05"));
  
  Serial.println(F("Connection to uradmonitoring platform Disconnected."));
}

Wireshark is not yet possible as my device is in a secured vlan.