If you're getting Access Denied, then the user you're logging in with simply hasn't been given enough permissions to access the SharePoint site. You'll have to login to the site as an administrator and add the user to a group with permissions (Site Owners, Site Visitors....). It could also be the url on the site you're trying to access. It might be to a Site Settings page that only site collection admins have access to (I noticed _layouts in the url, which made me think that might be why you're getting Access Denied). In that case clicking 'Go back to site' should take you back to the home page for the site, where the user might have the appropriate permissions.
Also, just an FYI, if you're getting this Access Denied page, then your membership configuration is correct and you shouldn't have to make any more changes to your web.config. If you had an incorrect web.config/setup issue, you would get an invalid username/password error instead of Access Denied.