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I figured it out finally.
The problem was NOT rp_filter. QmidiNet is even setting the proper multicast group via SNMP, buuuuut ...
This means it can receive multicast data but there is no one who can actually send it :D

I did a:
ip route add 224.0.0.0/4 table main scope link dev MIDI_NIC

on all sending machines and it works like a charm :)

My intention is now to make a small MIDI network monitor in a tcpdump | od | awk chain
once done I'll upload it here

Once more thanks for your input