Entering the serial number and email address produces 2 check marks, but the registration fails. The program seems to be working, but no option to activage the MP4 codec which I really need.
Turns out I had two versions of MX on my PC: plus and permium. The plus was a trial. The premium was the one I bought. They key from premium worked on plus (green check mark), but would not activate. It's ok now.