Hi BartE - thanks for your reply!
I do not receive any error regarding invalid EEPROM - not while writing, not at boot time.
My cards are shown in this format: D4 04 E2 E9
I register a Master Card like this (and it works until reboot):
MFRC522::Uid masterkey = { 4, {0xD4,0x04,0xE2,0xE9}, 0 };

Hello BartE,
I have the same issue as cyberfilou...
Sketch works beautifully...but if I power off the Arduino (Pro Mini in my case) and power it on again, all stored cards are gone. Any ideas what could cause this, and how to prevent it?

Thx a lot, Stefan