Beginning at about build 100 of J. River Media Center, DLNA support began to be more reliable. You will need to set the UPnP Server to "DLNA".
DLNA is a super-set of UPnP. In UPnP protocol, there are three components:
1. A Device (a server)
2. A Renderer (the client)
3. A Control Point (something like a remote control)
Our UPnP Server is a "Device" (#1 above).
More information on UPnP can be found in the article on UPnP Server.