Imagine if you'd gone to the actual ticket office this morning to buy tickets, they'd be a queue of folks wanting the away tickets available today. It's the same as that online. You just picked the wrong moment to try to buy tickets. Wait 30 minutes and it'll all be fine.
That's only on "release" days to stop folks trying to jump the queue. If there weren't a load of away tickets available today there wouldn't be the queue system in place.
Unless you're on a wind-up?