I recently encountered the same issue as reported by the OP. Web Services told me it's a known issue that happens very rarely and they don't know why. All will appear correct, but it just won't work. They also told me it happens more frequently on itineraries where the passenger is Executive Platinum, which seems odd. I was told that when this occurs, there is no known cure but to rest assured the reservation was perfectly in order (and it turned out to be fine).