QC didn't work on 6.1 after upgrade from 6.02 either. I used a real serial and mac-adress from an old Synology NAS.
Now I have generated serial and 2x mac-adress for DS3615xs and it works again. They don't check if serials really exist.
They may check if the serial / mac-adress match the corr. Synology NAS type.
Cheers