Ports needed to use Ondemand5.com are 80, 8080, 8088, and 443.
If you cannot gain access to the full site through any of the regular troubleshooting methods have the user clear their proxy (server) cache.
If they login and receive a blank screen, or are completely unable to log in even after a username/password reset, make sure port 443 is open on their router/server/etc.
If you do not receive the login prompts for username and password, please review this KB article: Can’t Login – Compatibility Mode