I have the same problem: When trying to publish with WLW I got the error Invalid Server Response… Looking into WLW logs I got: Warning: strpos() [function.strpos]: Empty delimiter in /home/user/public_html/mugur-ionescu.ro/wp-includes/class-wp-xmlrpc-server.php on line 2470
If I comment the line
if ( strpos( $post_content, $file->guid ) !== false )
everything works well but I am wondering what is the official fix.
For me the problem seems to be cauzed by the theme Pagelines (I found similar issues on Pagelines forum) but I saw similar problems caused by various plugins. Any clue?
Thanks