I’m not sure why you are having the problem. I have developed agents using Notes R7 beta and these have run fine on a 6.5 server.
When you got the “Page not found” error, what was the url in the browser ? It seems like the agent tried to do a redirection but something went wrong. This url should give you a clue to what is going on and may help you to debug the problem.
Let me know how you get on.