I can't remember the exact code I got when trying to use my new carnet tickets on FCC the other day, but I'm pretty sure it was 11x - and all of these appear to be relating to the date validity.
Very odd. Will be travelling again tomorrow and will see if I get another error. My tickets are valid from May 22nd to August 21st, so the gates should accept them.
The man at the gate just let me through and said he trusted me! Didn't even inspect the ticket, or check the code on the screen. How many members of staff are likely to know these codes?