Arduino Memory Issue

I am doing a project with Arduino.

Here is the piece of the code of one method I used

void HttpUpstreamClient::sendAlarmDeviceId(char* alarmType, char* alarmText, char* severity, String timestamp,char* URL, String deviceID ){

Serial.print("The alarm is sent to the device: ");
Serial.println(deviceID);
Serial.println(timestamp);

StaticJsonDocument<100> root;
JsonObject source  = root.createNestedObject("source");
source["id"] = deviceID;
root["type"] = alarmType;
root["text"] = alarmText;
root["severity"] = severity;  
root["time"] = timestamp;



String body2send= "";
serializeJsonPretty(root, body2send);
Serial.println(body2send);

if(_networkClient->connect(URL,443)){

Serial.println("Connected to server");

_networkClient->println("POST /alarm/alarms HTTP/1.1");  
_networkClient->print("Host: ");
_networkClient->println(URL);
_networkClient->print("Authorization: Basic ");
_networkClient->println(_base64);
_networkClient->println("Content-Type: application/json");
_networkClient->print("Content-Length: ");
_networkClient->println(body2send.length());
_networkClient->println("Accept: application/json");
_networkClient->println();
//client.println("Connection: close");
_networkClient->println(body2send);

}    

   }

The most important point I want to mention here is the parameter String timestamp.

In the main.ino, the part loop() is as following:

void loop() {
 // put your main code here, to run repeatedly:

 // from the server, read them and print them: 
  while (nwc.available()) {
  char c = nwc.read();
  //Serial.print(c);        
  }

timeClient.update();

delay(1000);


clienttest.sendAlarmDeviceId(alarm_Type, alarm_Text,Severity,timeClient.getFormattedDate(),URL,"2556130");

}

That means the method "Serial.println(timestamp);" can work. However, it doesn't work when it's value passes to the StaticJsonDocument<100> root.