HTTP status code "-3"

Projects General Guidance
4

Example:

  String postData = "name=Alice&age=12";

  client.beginRequest();
  client.post("/");
  client.sendHeader("Content-Type", "application/x-www-form-urlencoded");
  client.sendHeader("Content-Length", postData.length());
  client.sendHeader("X-Custom-Header", "custom-header-value");
  client.beginBody();
  client.print(postData);
  client.endRequest();

Your code:

  client.beginRequest();
  client.post("/v1.1/messages"); //, contentType, buf
  client.sendHeader("Authorization", AuthorizationData);
  client.sendHeader("Content-Type", "application/json");
  client.sendHeader("Content-Length", len);
  client.endRequest();
  client.print(buf);

No, it is not a server problem, your code is wrong. You're sending data after you ended the request, and you don't finish the header or start the body.