sudden strange error when attempting to post new post or save draft

So, I’m finally starting to “populate” my new blog. Posting is going along fine, when I go to publish a new post, and I get:

Not Acceptable
An appropriate representation of the requested resource /stage/wp-admin/post.php could not be found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Wot tha…?

Thanks in advance,