I was recently trying to book a flight on American Airlines using accumulated miles/points. Every time I got to the end of the booking process, I reached a page that said “This Page Has Taken Flight”, and I lost all of the booking information I had entered thus far.
Eventually I found the solution on a web forum posting. Apparently, despite having flown on American Airlines many times, my account had gotten into a state such that my home address was not on file. Entering my home address into my account details enabled me to complete the flight booking.
This error message was very mysterious, and nothing about it even remotely suggested what the problem was. Sharing this tidbit here in case it helps anyone else!