After an upgrade from WordPress 2.3.something to 2.5.1, my Windows Live Writer 2008 suddenly refused to publish any posts, just complaining about

The response to the metaWeblog.newPost method received from the weblog server was invalid:

Invalid response document returned from XmlRpc server

A (lot) of googling left me with a bunch of different solutions, ranging from .htaccess hacks, stripping whitespace from the xmlRpc.php file, disabling plugins and themes, and all types of stuff like that. None of which helped. One of the more useful tips was to find Live Writer’s log file (Help, About Windows Live Writer, Show log file), but that didn’t make it any clearer what was happening. (Apparently the XML file was missing a valid root node. Gee, thanks, now I know exactly how to fix the problem…).

Long story short, after trying to reproduce the error with some other blogs, it turned out that I had made a mistake while upgrading WordPress. I had merely overwritten the 2.3.x files on my web server with the corresponding files from 2.5.1, but to do a correct upgrade you apparently have to DELETE the wp-admin and wp-includes folders first. When I did this, Live Writer just started working again… I suppose some old deprecated file was left in the folders, raising hell with something (I’m not going to waste time debugging exactly what and why, since everything’s working now I’m content).

Since I hadn’t found this trick anywhere on the net, I’m writing it up here for all to see (provided Google indexes this blog someday).