Mifare Classic Tool error rewriting dump

Hi,

I'm a total n00b to the NFC world so I apologize if my message is not clear or lacks important information (I'm happy to answer any questions to the best of my abilities).

I use the Android app for MCT (currently version 4.3.1).

I created a dump of a Luxéo keyfob (for a vending machine) a few months back, and have been using it regularly, rewriting the dump every now and then.

Suddenly, today writing the dump failed, with the following message: *Sector X: Keys (A and B) not known (*with X going from 8 to 15).

I tried extracting a .key file from the dump, but that didn't work. Don't know what else to do.

If anyone knows what can cause this and how to fix it, it'd be much appreciated.