Hi @JozsiKa ,
I am not a BLE DFU FW update expert. But if the board is working previously with the default firmware, We can assume your modification caused the BLE DFU errors.
I saw your post here as well - RAK5010 Firmware replacement, debugging with RAKDAP1
@beegee already gave pointers there so you can recover your boards to have the default FW and try to play again with it. I suggest you start first by recompiling the default RAK5010 firmware and upload your new code via BLE. If it works, you can then add the modifications little by little until you see what possible causes it. For now, let’s try to make your device fresh again.