Why do Chatropolis entry pages give "403 Forbidden" error on all browsers?

I've been running Mac OS 10.4.11 successfully since it's release, and I normally have the following browsers installed & working: Safari 3.1.1 (my default/first choice), Safari 2.0.4, Firefox 2.0.0.14, and the PC version of Internet Explorer (6.0.2) which I can run in Windows 2000 under Virtual PC 7.0.2 emulation.

Suddenly, 2 days ago, for no reason at all, the ability of my Mac to access any of the chatroom entry pages on the chatropolis.com website stopped working. (Please be warned - BEFORE you visit chatropolis.com - that it primarily hosts ADULT chatrooms!). This is after many years of problem-free access to these pages, using my current Mac, and previous Macs.

I can still access the home page, the user list pages, etc., but trying to access the room entry pages for any chatroom at all gives a simple black page with white text, which reads:

403 Forbidden
You are not authorized to access this resource.

____________

The entry pages all take the form http://csX.chatropolis.com/enter/roomname

where "X" is the server number that the particular room is hosted on (normally "7" or "10") and "roomname" is the name of the chatroom that I'm trying to enter. For example, the URL of the room entry page for the room "Current Events" is http://cs10.chatropolis.com/enter/currentevents

I can access any page on the Chatropolis site that DOESN'T start http://csX.chatropolis.com/enter/....... but none of the pages that DO start this way!

This occurs whichever browser I use, and even occurs using the PC version of Internet Explorer 6.0.2 running under Virtual PC emulation!

I've tried all of the following to solve the problem, but nothing works:

I've tried deleting both Safari 3.1.1 & Safari 2.0.4, and then reinstalling Safari 3.1.1.

I've purged all the caches, cookies, browsing histories, etc. of all my browsers, both within the browsers themselves, and by using Intego Washing Machine 1.1 (which comes free with Intego NetBarrier 5). I do this purging regularly anyway.

I've restarted the Mac several times.

Using Disk Utility, I've repaired permissions on the boot HD several times. I've verified the boot HD, and no problems were found.

I've rebooted the Mac from my Alsoft DiskWarrior 4.0 CD and successfully run "Repair Disk Permissions" and "Check All Files & Folders" on all my hard drives, including the boot disk, and I've successfully rebuilt & replaced the directories on all the drives.

I've successfully accessed the pages in question using a friends Mac, who lives very nearby, and uses the same product (2 MB cable broadband internet connection) from the same ISP as me.

I've searched for all files on my Mac that have a modification date of 2 days ago, and deleted any that are in any way relevant to web browsers.

I've emailed the owner of Chatropolis (Michael Ludwick) and asked for his help. He thought that maybe I'd installed some software prior to the problem arising, but this isn't the case. He also dismissed my theory that my ISP might be blocking access for some reason. On the day I first emailed him (Sunday), he told me that there had been over 700,000 successful logins, and that I was the only person to contact technical support that day saying that I couldn't gain access. He tells me that "The error page actually resides on your machine so the src isn't important" even though the colour scheme of the error page matches every other error page on Chatropolis!

I now suspect that the problem lies in the part of the Mac OS which is activated when certain types of pages on the internet are accessed, whichever browser makes the call - some deep part of the OS that is normally invisible to the user, and has to function normally for certain types of internet access to take place.

All non-entry pages on Chatropolis work perfectly, as do all other websites, as does sending & receiving email.

Everything is bang up-to-date with Software Update. Nothing was installed or removed prior to the problem occurring.

If anyone has any suggestions, I'd be very grateful. I'm no UNIX geek, but I've been using the Mac OS since the System 6 days, and I know my way around it pretty well. I'd rather venture into the dark abyss that is Terminal and perform some sort of command line fix than have to reinstall the entire OS!

I've read many times over the years that Mac OS X performs housekeeping/janatorial tasks "overnight" from time to time, but I never leave my Mac on overnight. It's shutdown & restarted at least once a day. Is there perhaps one of these "housekeeping" tasks that has never been performed, and that should be, that might solve the problem? If so, how do I force these tasks to be performed immediately, to see if that helps?

Many thanks in advance for any constructive input!