If you keep getting “Invalid Server Response” error while trying to publish a post to WordPress from Windows Live Writer possible culprit could be one of plugins installed on your site and in my case it is Smush.it. If you’ve not made any changes to your site recently, to troubleshoot this error I recommend you to disable installed plugins one by one and then try to publish once after deactivating a plugin. It took me months to figure out Smush.it from Yahoo is root cause for getting that warning, could be different in your case.
Here is the what Invaid Server Response dialog says
“Invalid Server Response, The response to the metaWebblog.newMediaObject method received from blog server was invalid. Invalid response document returned from XmlRpc server.” Check the screenshot below.
Yesterday after upgrading to WordPress 3.5 I couldn’t able to publish a single post to my blog with images included in the post. While playing with images on WordPress I got an error related to Smush.it which when I disabled it, I was able to post the article without any issues.
XML-RPC enabled by default in WordPress 3.5
And one more: From WordPress 3.5 onwards you don’t need to enable XML-RPC to remotely publish from desktop blogging clients such as Live Writer because it is enabled by default now. And XML-RPC option used to exist under Remote Publishing for Writing Settings section has been removed now.
Say one reason to use Live Writer
From early blogging days I used to publish posts to my blog using Windows Live Writer, recent times its not changed. If you ask me say at least one reason to use Live Writer, here it is: one comfort with this tool I can say is you can easily paste images directly into the WLW where uploading or dragging and dropping images is required for WordPress.