is that your have your port fully invested ($0 cash on a regular basis)
and last night you dumped some stocks to free up money to invest in CARS2?
and those dumpings did not clear before your CARS2 order was fulfilled (meaning that technically, you had no cash at the time HSX tried to process the order) so it failed
In my fund, I try to be fully vested and when I dump stocks to pay for other stocks I always refresh my port a few times until I am sure the dumpings have been processed, then I take the money and invest it and that way I do not have any problems.