Hmmm, so you're saying AC picks a threshold based on how it values its flights (in Aeroplan miles), and if it's above it, it won't let UA offer them on UA awards (at any value)? Learning something new.
I find it hard to believe it's an inventory thing, because according to the UA search, there were 0 award flights available, at any point value, any day of the month. That's why it looked more like a site error or FF dept error...