Had a similar issue a week weeks ago. Entered the codes, vouchers were validated, on final page got the "Session expired" message.
Had to wait the remainder of the 15 mins when the vouchers automatically refunded and then was able to successfully book.
This was for Captain Marvel a couple of weeks ago.
The past weekend when trying to book for Avengers... kept on getting bad gateway error messages for the website. Luckily I passed Canal Walk so booked at the cinema using the codes.