Hi tomski!
Very short:
3 ports:
1. TCP
2. UDP port number is TCP port + 3
3. UDP
What happens? You try to set UDP (3.) to the port number which is already occupied by UDP (2.). Thus aMule has to set the port numer for UDP (3.) to something else (free).
Set UDP (3.) to something else, e.g. 4672 and it will work.