Okay, after some further experimentation, I've continued to have problems. I have not seen the freezing issue again, so maybe it was related to something that went away on reboot or something, but there seem to be consistent issues with playing to a Bubble DLNA renderer from JRiver MC. (Edit: it did freeze again, but may have been related to killing the DLNA renderer that was the actively selected zone.) Here's what I see.
Playing from JRiver MC to a Chromecast Audio wrapped by a BubbleUPnP Server DLNA renderer appears at first glance to work. But one thing fails every time: when playing a full album, if at any point during the playback of a track I click to a different spot in the current track timeline, it will successfully jump to the new spot, then play out the rest of the song as expected...and then stop. The next track never plays. This happens 100% of the time, as far as I can tell. This has been advertised as a working solution here, so maybe someone has gotten it working...or maybe it has never been explored enough to notice this issue. I can't really go with a solution that is so fragile as to stop every time someone navigates within a song, so I'd love to get this resolved. In case there are clues within, I'm attaching a log file that covers the steps of playing an album, jumping to near the end of the song, then letting it finish and fail to go on to the next song on the album. If the fact that I'm playing from a remote library adds too much complexity to the log file, I can create another one where the files played are local ones, since the problem occurs either way.
For comparison purposes, here's the behavior I see with other applications.
1) Windows Media Player playing to the BubbleUPnP Server DLNA renderer: Moves from song to song in a playlist just fine, regardless of whether I've jumped around in the currently playing song. So, not showing the same problem. (It has other problems, though-- I can't actually navigate track to track manually, only by letting one finish and the next begin. In fact, if I try to jump to another track, it weirdly shows the new one as playing, and the time indicator counts along as if it has worked and I'm now starting playback of a new item, but the old track is really still playing.)
2) VLC playing directly to a Chromecast Audio device: seems to work perfectly, at least so far.
Is there anything else I can do to figure out clues here, or does anyone else have JRiver MC talking to a Bubble server who could try duplicating my issue? Thanks.