Saving Edits Fails

Periodically, my WordPress install simply refuses to save edits to posts, or, alternatively, it saves them but still throws an error message. Sometimes, but far less often, it will refuse to save a post (except it usually does it, simply throwing the error but going about its business as usual)

I can not predict which edits will produce the error and which will slip by, unnoticed by the script…

The error is:
Not Acceptable
An appropriate representation of the requested resource /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.

It makes absolutely no sense to me, at all. If it happened every time, then there might be some logic to it, but it doesnt.