I'm getting this error too but my site never really worked right. Brand new clean install, no mods, 1 style not currently active.
Order of Events:
1. Installed directly on the server. Windows 2000, Internal address 192.168.2.55.
2. Everything seemed to be working but I was browsing http://192/168.2.55/
3. Installed the AutumsColor style.
4. Next day accessed the site from home using the public address http://www
dot thegalleryisle dot com.
5. Seemed to be working until I tried to access the ACP and it kept trying to redirect me to 192.168.2.55.
6. Logged into the server remotely and got right in using the internal address.
7. Changed the Server settings to force server url to http://, www dot thegalleryisle dot com, 80, /forum
8. Tried again from my home browser (Win7RC, IE8). Redirection was correct but started getting the not allowed message. Clicking on the index page after the failed attempt showed I was no longer logged in.
9. Tried again from the server and found I get the error using the url but still get in using the internal IP.
10. Backed up and purged my database and restored the install directory.
11. Installed from my home pc using the url instead of from the server using the IP.
12. Could not get into ACP from home or from the server even when trying the IP.
13. Purged database and restored from backup.
I used phpBB 2 a long time ago without any problems. I would be a real shame if they broke a good product. It is totally unusable for me as it is now. I can't always remote into the server and use the IP address when I need to administer.
--- Edit ---
Updated results... I managed to fix my own problem. It seems that the firewall (ISA Server) needed to have the web publishing rule set to forward the host header on to the web server.