I should also state that exactly the same behaviour happens consistently when using Bubble to control MC on PC1 to play to itself. It is not related to the library being on PC1 and the other player being on PC2. I tried to use the Whitebear program to run another set of tests on PC1 only, but it reported the Associated MC DLNA Server as "<FORBIDDEN>: Media Center 21 Renderer, from Media Center 21 ( on this computer )".
Aha.
Just an explanation about this "FORBIDDEN" message..
Of course it is
allowed to use UPnP/DLNA functionality to push a track from a library server (DMS) running on one instance of MC (running on one PC) to a remote media renderer (DMR) running on another instance of MC (running on a different PC).
But it makes no sense to use UPnP/DLNA functionality to push a track from a library server (DMS) running on an instance of MC to a media renderer (DMR) running on the
same instance of MC. The reason is that MC can spool the music samples internally from its library directly to its audio driver output, and so there is absolutely no point choosing a roundabout route of pushing the track out of its DMS back into its own DMR. Indeed MC actually hides its own renderers on the same machine, to prevent the user from even contemplating doing that. Therefore my DMRA test application reports such a combination (MC DMS pushing to itself) as "forbidden".
The fact that you mention this point, makes me wonder if you are indeed trying to use Bubble as a Control Point to command MC to serve tracks from its own DMS to its own DMR. Can you please confirm?
If that is the case (MC serving to itself), then it is indeed quite possible that you may encounter errors. Because as mentioned above, MC is not actually designed to serve to itself, and certainly I doubt that anyone at JRiver would have evaluated such a test case. I can imagine several possible problems -- namely a) at the network level it means that the same network card is opening a TCP socket to another port on the same network card, and there may thus be pipelining issues within the network card driver, b) such self routing behavior might indeed arouse the suspicions of a firewall or a/v program, c) if the Bubble CP is also on the same machine as MC then the TCP routing might be done via the loopback IP address 127.0.0.1 rather than the PC's external IP address, and again depending on your PC this may or may not work, d) if the TCP routing is indeed going over 127.0.0.1 then the renderer's NOTIFY messages (which are UDP mono-casts) would also be routed via 127.0.0.1 and so may also be suffering from routing anomalies, and last but not least e) it may be that MC's server and client threading model may not be designed for serving to itself, and that the two threads may be blocking each other and (presumably) hitting time outs.
The potential problems a) thru d) are network / routing issues that JRiver would not really be able to solve; whereas the potential problem e) may be something that JRiver could possibly address.