+1.
I used to be able to play both 2ch and mch tracks of a mch SACD .iso on a MacBook Pro running jriver 19 and earlier versions of 20 (for Mac, of course). The output was configured as a hi-res (usually 88.2 KHz) 5.1 channel signal via HDMI to my multichannel processor, which reported correct sampling freq and channel number.
NOW, after I experimented for a while with a mch USB Dac and JRiver, i abandoned it in favor of a Marantz AV8801 processor (again, from MBP via HDMI), and cannot get the mch versions of .iso tracks to play in any kind of PCM format. Exactly like the original poster, I get a long period of "buffering", and then "something went wrong with playback" message. Of note, stereo SACD-derived .iso files play just fine, and for the most part 2ch versions of tracks from mch .iso files also play fine.
Please advise, as the loss of ability to play mch .iso files in JRiver 20 is majorly screwing up my attempted transition to file-based playback. And yes, I know mch .dsf files can be extracted but I have a rather large sacd collection that I ripped to .iso and cannot see going through the time and space waste of .dsf extraction.