Did you pay for the 8/9 first and then tried to purchase the 9/9 ticket? The code probably only allows one transaction..
I tried to pay 8/9 first, but that payment failed (both the local iDeal option and my credit card failed server side).
I had two browser tabs open (one per day) and that is apparently a stupid thing to do. When I tried to pay again the 8/9 ticket was changed into a 9/9 ticket (from the other tab?).
However, it seems like that my code worked again 1 hour later (perhaps the failed transaction was no longer reserved at that time?) and I was able to buy 1 ticket for 8/9 after all!
Now I just hope they adhere to the policy on u2.com/help and allow both transactions (each 1 ticket, with same U2 presale code, but different order numbers for tickets due to site errors).