Originally Posted by Adam Smith

You can only check in for a flight around 48 hours before it's scheduled to depart. If you were trying to check in nearly 24 hours before your first flight, and you had two connections and a very long layover, I wonder whether it was erroring out because it couldn't check you in for the last leg because you were too far out. A few hours later and the window for that segment might be open, eliminating the problem.