Originally Posted by
Simon
It would appear you have a case and the flight should not have been closed, particularly with you already having checked in:
"Checked-in" means completely checked-in. Since it would be impossible to fully check-in online with hold baggage, the OP had to complete the check-in process at the airport, where (s)he had, by their own admission, missed the check-in deadline (30 minutes, i.e. 06:15).