HP-UX?
Well, it could be a lot of things like:
a firewall,
a proxyserver,
bad rewriting using mod_rewrite, etc.
I need a little bit more information.
What are the apache log files saying, there should be some traces of POST. If you are running a firewall like iptables, what does its log say. If you are using mod_rewrite, turn on the logging feature.
Good luck,
Marvin