I'm trying to book a 13:30 train from Strasbourg to Nice, fr but when I click the review order button online I repeatedly receive a message saying "one of the legs of your trip could not be booked." I don't understand what is causing this message to appear and why it's preventing me from reserving my spot on the train. Though the train is schedule for later today, its departure time is still more than 3 hours from now, and the website said many open seats were still available. Why is this message appearing? I've tried 3 or 4 other booking too and all gave me the same message.