Yes, it’s definitely a 3.6 issue for me too. I’m not using Windows Live Writer, but every client that I’ve tried comes back with the same error.
I think given that we’re all on 1&1 it’s something in the way that 1&1 servers are configured. I’m clueless as to what it could be though – it’s probably something that changed in xmlrpc.php in 3.6.
Investigating further, using Marsedit, if I’ve created a post using the web and hit the refresh button, it pulls down the new post – but it then gives me the error. I can edit existing posts and send changes fine. I can create a post and send it, and it’s posted OK. It seems like it’s only when the client asks for a listing of posts that it gets the error, and the error appears only after it’s pulled down the list.
Bizarre.
