@dc42 If I get it to be recognised as USB Serial COM-port, and connect to it with YAT then it responds to M115 and M122 (booted up without a SD card present).
Just now, I tested this by connecting the board without SD only to USB and pressed reset: no port was shown, diag LED stayed on. Then, while typing my reply, a 3 or 4 minutes after reset I see that de Diag LED turns off and a USB serial COM-port appears in Windows. Never seen that before that it took so long.
I tried again with the SD Card present resetted and waited ten minutes, Diag LED stays on. I tried again without SD card, waited and now the led stays on.
So I find it's behaviour unpredictable and don't know which component could broken.