OK 3 months and still no replies.
Thank you for your posts, and I apologize for the late reply.
FileMaker runs on top of the system, so if the system is truely filtering out certain IP Addresses, the system should take precedence.
Try setting up a dummy web site with Apache and see if that IP Address is able to access the web site. This may help with further trouble-shooting.
I have access to a OS X Server 10.5.8, but after ten minutes of searching, I don't see where the IP Filtering is located (I expected it to be under Security -> Firewall). Any help for me to replicate the problem may be helpful.