Hello Ryan,
I too am having the same problem as you with the same router (v01) and same firmware. After months of googling to no avail, I think I found a solution. Being a web developer myself, this is obviously a bug with the built-in web-server for the router's GUI.
I tried everything from compatibility mode, to just about every browser, and fiddled with the html code using firebug. So heres the problem, when you ADD a new CUSTOM service, and then attempt to add it to the list below, it brings you to a blank cisco page. As a matter of fact, for me, even adding one of the default services brings me to a blank page. This started happening after about 3 years of daily use. Now, even factory reset to an older firmware version does not make it work.
One thing I noticed, when you are on the blank page, you can go to the URL bar and press ENTER with the URL still there, and it will bring you back to the UPNP page and process your request, but only if you add a built-in service it will work, IT WILL NOT WORK with CUSTOM service.
Interestingly enough, after days and days of trying just about everything possible, I noticed sometimes, some custom services will go through, mind you, there is no avoiding the blank page. It will ALWAYS happen, but if you press ENTER at the URL bar with the URL it shows you, it WILL go to the next page.
So, to make it short, for me, what makes custom services work and lets them be added and enabled, is to add a DUMMY custom service ALWAYS AT THE END OF YOUR CUSTOM SERVICES LIST.
For example, say you want to enable port 8000, ADD the service for 8000, then after that ADD another service with a useless port, you do not have to enable it, just add it to the end of the list. It seems like the web configuration will not ALLOW the LAST service on the list to be added at all, but you can add the one above the last one. Doing this is how I have been getting by and adding services to the UPNP list. Remember, you will always get the blank page, just press ENTER on the URL bar and your request will go through properly, well, at least this works for me, same router, same problem same firmware.
The URL should look something like https://IP.IP.IP.IP/adv_upnp.htm
I hope this helps you out, I registered here only to reply to your problem because I too had a very hard time with this and I hope other people out there experiencing the same problem can solve this using above method. Cisco is clearly not interested in fixing this problem.
