JRiver MC 28 + BubbleUPnPServer (0.9-update41) + (player: JRemote2 or Panel or Android BubbleUPnP or MC 28) + (original Chromecast Audio dongles as DLNA renderers) worked perfectly for the last month, until Chromecast with DLNA mysteriously fell apart a few days ago (discovery worked to some extent, but no music played). I tried most things imaginable to fix it, some many times over - nothing helped.
Fix: A few hours ago Bubbleguum posted a new version to address a recent firmware change in Chromecast Audio: BubbleUPnPServer 0.9-update
42. Worked first time out of the box! The update can be applied from within the Bubble web browser configuration page > Settings tab > Update tab > "Check for update".
FYI: During this ordeal, Whitebear DMRA discovered ok, but exhibited a timeout error and no sound. After 0.9-update42, no timeout and it plays a very welcome sound. I also found that Android BubbleUPnP App 3.5.8 served MC 28 libraries to non-DLNA chromecast audio renderers normally throughout.
References: https://groups.google.com/u/1/g/bubbleupnp/c/5k_zagk0owEhttps://www.googlenestcommunity.com/t5/Chromecast/Chrome-casts-not-working/m-p/59944#M6393Chromecast Audio ("Second-Generation", GA3A00147-A14-Z01, Model RUX-J42) new Firmware Version: 1.56.281627