“WindowsLive.Writer.CoreServices.XmlRpcClientInvalidResponseException: Invalid response document returned from XmlRpc server —> System.Xml.XmlException: Root element is missing.”

This make me believe that one of the components on your site is throwing a PHP Warning or Fatal error message. You should take a look with Fiddler (or any other HTML traffic capturing tool) in order to resolve this issue because you will get access to the full response returned by the server and not just the little part written in the log file by Windows Live Writer (WLW).

On my blog, I’ve recently posted the part 1 of an article on how to use Fiddler to resolve some of the problems between WLW with XML-RPC:

http://coding-paparazzi.sylvainlafontaine.com/2010/02/solving-connection-problems-wlw.html

The warning and fatal error messages from PHP are not yet covered (this will be the subject of the part 2); however, the instruction on how to use Fiddler with WLW and XML-RCP could be useful to you for solving this problem.

For other people having one of the other problems between WLW and XML-RPC, the subjects covered in this part 1 of the article – beside on how to use Fiddler – are the presence of a BOM at the beginning of one of the PHP file (most often this will be the file wp-config.php because this file is often edited post the first step of a new installation of WordPress but it could be another one like xmlrpc.php if you have played with these files) and the presence of extraneous characters at either the beginning (before the opening tag) of the PHP file.

Also, did you get this problem since the beginning or did it just suddenly pop up?

Try using the default unedited theme that came with WordPress without any plugins just to be sure.

But I still feel a PHP upgrade would almost certainly solve your problem.

Thanks for your help. I am going to try the other 2 steps you mention, but I believe I did the deletion of the spaces already.

1. Your webhost.
2. PHP Version (If you can easily find that out if you have the Security Scanner WP Plugin). You can get that plugin from here if you dont have it already:
http://wordpress.org/extend/plugins/wp-security-scan/
3. List of plugins.
4. WLW version.

Some solutions that you can try:

1. Delete spaces after “?>” from each of the files below:
xmlrcp.php
wp-config.php
wp-blog-header.php
wp-settings.php

2. Disable all plugins and enable them one by one and see if maybe one of them might be the culprit.