I'm getting a "Page not found" error when trying to access the internal area of the website.

Please go back to the page with the link that you followed. Click "sign in" at the very bottom of the page. The URL should change to "sites.google.com/...". Now access the link to the internal area again.

I'm having problems using the cost-ic0701.org account and my personal/other Google account on the same computer.

Please enable multiple sign-in for all your accounts.