I'm ready to leave 1and1; their support is simply atrocious. Wasted 30 minutes on the phone yesterday with a clueless support engineer who was of zero help.
I've tried a few other things with no luck. I added the following code to the .htaccess file on the root of my wordpress folder:
<Files xmlrpc.php>SecFilterInheritance Off</Files>
But this actually broke it completely, triggering a 500 server error for every attempt. So I reverted and I'm back to the 503 errors.
What makes me think this is related to request size is that when I use the 'Open Recent Post' option in WLW and then select my blog, the request to parse the list of articles on my servers works almost exactly 50% of the time. Performing the same requests, but asking for different sizes of article lists (20, 50, 100, etc) will intermittently succeed or fail, and the request size doesn't matter. Sometimes 20 will work and 50 will fail, other times 20 will fail, yet 100 will work. So I'm convinced this is not some type of 'something was switched off' problem.
I also am able to create a new article in WLW and then both post a draft and publish the article. When doing these steps the 503 will happen sometimes, but as long as I keep hitting post it will eventually succeed (typically after 3/4 attempts). But downloading a saved draft from the site into WLW still fails 100% of the time.
Can I assume you guys are also seeing this as of WP 3.6? This worked fine in 3.5.2 and earlier.