I've experienced the above issue. There are some strings that, when sent through [POST], are not passed to the php engine as they should and cause apache to kick instead. It's most likelly an issue with apache. This is a bug that should affect every account posting at any thread, as long as the post contains a "bad" string.
Inside your post smurf, there must be one of those strings that triggers the failure you experience. Do you think you can try and issolate it as much as possible? Or mail me the message text at nske[AT]firewall.cx and I'll try to find it.
Thanks & sorry, I hope you didn't lose much time & effort because of this stupid bug