I booked a flight on the KLM site several months in advance and got the line: "U kunt niet online inchecken voor deze vlucht."
(You can't check in online for this flight.)
The flight is on a national holiday and I was pretty much sure at the time that the reason for not being able to check in online was that holiday.

Is there a way out and still be able to check in more than three hours before the flight?