So I went on the co-op queue, it was a slow morning and it did not start at once, and I opened the auction house.
And three times in a row, the join co-op crossed swords appeared on top of my auction house page, I clicked join, and the co-op didn't start... although other people went off the queue, so I'm guessing the co-op fight did occur, just without me.
Could it be that looking at the market price somehow prevents you from joining a co-op ?