after some google-fu:
You could try unload module nf_conntrack_h323 if you dont use it.
Or if you cant live without this module try to contact with module developers at http://nath323.sourceforge.net/ . They's module generates that error, probably module bug. At least they can tell what tool to use to debug with error.
This module used by netfilter nat and/or statefull firewall to work around with VoIP h323 protocol. So if you dont nat any h.323 users just unload this module. Most VoIP applications can use sip VoIP protocol too which is prefered anyway.
only applies to kernel 2.6.x