I was getting this when using UPNP plugin > BubbleUPnP renderer (running on server) > Chromecast Audio (on WIFI LAN)
Sometimes BubbleUPnP wouldn't respond, after a server hibernate/wake up etc. and MB would give this error as it's default player output wasn't available.
BubbleUPnp has been updated and I haven't seen this today, whenever it did I had to restart the BubbleUPnP server, this woke up the renderer and MB would cast again to the Chromecast Audio.