After upgrading to WordPress 3.1.3 i get this error when I post news articles via Windows Live Writer:

“Invalid Server Response – The response to the metaWeblog.newPost method received from the weblog server was invalid: Invalid response document returned from XmlRpc server.” .

The error in the log is

PHP Warning: strpos() [<a href=’function.strpos’>function.strpos</a>]: Empty delimiter in /usr/home/zoso/www/ on line 2379

I just removed the line 2379 from wp-includes/class-wp-xmlrpc-server.php

if ( strpos( $post_content, $file->guid ) !== false )

And everything is back to normal.