Indeed, there are 6xKL, 4xLX, HV and U2 flights on Friday, it's hard to imagine KL could not find a seat on any of them and any connecting flights.
OP might be talking about KL1962 from Sep 21st,
https://www.schiphol.nl/en/messages/...september-21st
It would seem that the airplane that was supposed to operate said flight was redirected to operate AMS-ATH-AMS instead, which should allow a fairly straight forward claim.