I don't see how disabling IPv6 could help with IPv4 multicast issues, or indeed any IPv4-related issues, but OK; you tried it and it didn't work.
Have you checked whether the problem is on the sending or the receiving end? Run this tcpdump
command in a console on both nodes:
tcpdump -v -i interface host 184.108.40.206
with the correct interface name (bond0, bond2 or whatever). Then start the multicast test and see what happens.
Which Oracle version/patchset are you running?
Have you seen this