Theres a couple of things you could check -
1. all cgi scripts have execute and read permissions - otherwise it definitely will stop web browsers getting at pages
2. there could be a problem with the included httpd web server used to serve pages (I wouldn't be at all surprised given how "good" the ASUS firmware has been overall)
3. If you can switch to the custom firmware, I would - it gives you much more control over how scripts etc are executed since you can configure a different webserver