"Access Denied" means that the current user has been restricted by an Access List (or by Control Panel settings for Guest/Registered Users) from viewing the location they are requesting.
If you are seeing this for an area a user DOES have explicit access to, then there are three probable issues:
You have the "A user certificate is only valid if it comes from the same IP address" in the Registered Users Control Panel turned on, and your user(s) are coming in from an ISP that utilizes a Rotating Proxy Server. Deselect this option.
You have custom macro(s) that are losing the user's certificate. When this happens the state of the user can be lost, especially if your site does not use cookies (or the user has deactivated cookies in their personal preferences).