It's embarrassing, but I'll share the details. The end date (not the signup period, but the actual end date when the entire group buy should become no longer available) was accidentally set to July 1st instead of August 1st, so as soon as it reached 8 AM PDT here, the group buy page became no longer available. Quick and easy fix, and thanks to everyone who quickly caught and reported it this morning, it was only unavailable for a little under two hours.
357 more people until we reach the highest discount tier!