What causes XML Parsing Error: no root element found error?

Question

I recently started getting the following error displayed in Firebug:

XML Parsing Error: no root element found Location: http://dd.example.com/1.0/sources/30 Line Number 1, Column 1: 30:1:1

What causes it and how do I fix it?

Headers tab:

Request URL: http://dd.example.com/1.0/sources/30
Request Method: PUT
Remote address: 11.11.111.111:80
Status Code: 204 No Content
Version: HTTP/1.1

Request Headers:

Host: dd.example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:51.0) Gecko/20100101 Firefox/51.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://dd.example.com/
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 33
Cookie: _ga=GA1.2.1885184115.1475238275; PHPSESSID=7ggri150scu2g5et1f7gtone94
Authorization: Basic TEljaGGlcDqTdW5zaKfrZX==
Connection: keep-alive

Request Body:

name=name&value=Remote+Sock&pk=30

Response Headers:

Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Connection: close
Content-Length: 0
Content-Type: 
Date: Sun, 05 Feb 2017 16:59:50 GMT
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Pragma: no-cache
Server: Apache/2.2.15 (CentOS)
X-Powered-By: PHP/5.

Answer 1

正在发送状态代码 204，但仍在尝试发送内容。