Can’t read out some blocks

[CloudsEder]

Hello everyone,

so I wanted to use a MIFARE Classic Card to store a JSON in plaintext and read it using a stm32f105.
It works for most parts but not all.

I wrote the JSON on the Card using different Apps (NXP TagWriter & NFC Tools). When I read out the card with my phone it gives me the exact JSON I wrote.

When try to read it blockwise using my STM32f105 and a PN532, I get the correct values for every block besides 1. Block 21 always says "0xEA,0x00,0x00,0x00,..." and the text that should be in this block is nowhere to find. Block 20 and 22 show the expected values.

Does anyone have a clue why it behaves like that or a tip how to solve this?
Thanks in advance!

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[CloudsEder]

To add to this:

This happens with multiple different tags. All have this faulty readout of block 21...

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