Guess is: timing error due to packet sizes. Doesn't seem to be it... So what is the problem? It is very unpredictable... a noise thing? So I got it to go a bit further. Now 3 cards run up to: C-APDU:002C000100 UNBLOCK_PIN R-APDU:63CA And then things freeze: one byte is received (CLA = 0) and waiting for the next 4 bytes. No resets, no VCC off, nothing..