Thanks for the info. The production computer only has a single NIC, and we've tested the display computer with the second NIC disabled, but it didn't change the issue. Windows firewall is turned off at the service level, not just disabled, so it's unlikely that's the problem.
In the Wireshark traces I do see a multicast packet go out as soon as the secActivate command is sent, but the multicast packed doesn't show up in the traces on the display computer end. I suspect though that this is simply because the display software hasn't joined the multicast group.