Thanks for the information, whooami.
In case anyone else has this problem, I found a comment in this post which pointed to adding “SecFilterEngine off” to .htaccess which solved my problem. If you don’t want to leave mod_security off, you can remove the line after the post is published, and the post appears to be preserved. This article and this wiki page give more information selectively disabling mod_security. Another common error message produced by this problem is: “Error 412: Precondition failed”
