I had a hiccup yesterday afternoon where I got a message saying 'Access Denied'! But when I tried a couple of minutes later I got on no problem. Don't know what that was about.
Edit: Just done it again.
'Access Denied (policy_denied)
Your system policy has denied access to the requested URL'
Usually I get error 404 or 501 or something to that effect. Not always the same thing, but that seems to be worldwide when it happens.
Fortunately it seems to be very temporary.