I've posted this on the WLW forums as well, but it made sense to have it on both "ends" of the problem.
I'm running WordPress 2.1.2 on one blog, and 2.1 on another. Windows Live Writer works perfectly on the 2.1 blog, but has a problem with the other.
Whenever I post I get an error, "The response to the metaWeblog.editPost method received from the weblog server was invalid."
Despite this, the post still appears.
I realize it's most likely a WLW problem, but I have hopes someone from the WordPress side of things will know what's changed in 2.1.2 to cause this.
(I've also had the same problem posting images with WLW and 2.1.2, but found a workaround.)
Thanks for any help!