You're updating inside Wordpress, right?
Try this:
If your new post admin is located at say
http://www.SOMEDOMAIN.com/wp-admin/post-new.php manually change it to drop the www so it reads
http://SOMEDOMAIN.com/wp-admin/post-new.php. If it's showing without the www by default, then add it instead.
See if it will accept your post like this.