Yes - I'm getting that message. A bit of a surprise. Lucky i put a contingency plan in place a few weeks ago!
I had same message for Shrewsbury earlier in the season. I think the system releases so many tickets at once and if they sell out overnight it shows as not available. More get added the following morning. I'll be surprised if FGR doesn't sell out though, especially with it being a ground we've never played at before.