I have the same problem too. I have booked my tickets from Trip.com and I have just enquired about it with the CTrip agent. Well, they told me the ticket was totally fine on the website of CP. I have no idea about why my tickets were valid about 5days before but invalid now. Maybe this is an technological problem of the website of CP? Hope it would but solved asap because I'm about to have my exams and have no free to check all the news about it.