Just wondered what the problem was yesterday (Sunday 4th Dec) as my Forum was not accessible for a number of hours and the You must be logged in to see this link. website was unavailable (would come up with page not found, then later on with a 'service unavailable' message when typing in You must be logged in to see this link. in the browser.
Pleased to say though that now everything seems to be back to normal.
I'm still investigating the cause but it looks like the firewall was to blame. It had been blocking some legimate traffic over the last few days and I had re-configured it but eventually it just went belly up and decided to block everything even though it was disabled at the time.
The "Page not Found" error was caused by the firewall blocking you. The "Service Unavailable" error would have been because I was restarting the server.
Normally I am notified straight away when a server stops responding but wasn't in this case as the server was still up (just not serving requests). I discovered it first thing in the morning and fixed it within about 10 minutes. Since then I have been responding to email. I have taken appropriate action to ensure that I am notified immediately if it happens again.