We had a similar problem with the same configuration. It turns out that a Unity Registry key (RTPPortBase) somehow (sinister forces, maybe) got set to zero. As the name implies, this appears to be a base address for Unity to use to assign UDP ports to incoming RTP streams. This value is incremented by 2 for each new stream, until RTPPortBase+28 has been used, at which point the cycle starts over again at RTPPortBase. (All this is guesswork on my part, gleaned from examining Ethereal traces. I suspect this value may be different in larger installations.) Thus, in our case, every 14th message failed, since Port 0 is apparently ignored or otherwise invalid. I have no idea how RTPPortBase came to be set to 0, or how long it's been that way. TAC advised setting it to 0x5910; that fixed the problem.
