Many Yamaha devices have seek broken so it can fail when migrating an active playback to those devices.
From the logs seems the devices may have issues with gapless too, so disable the option in settings too then try again.
Since the devices accept the play command the possible last reason is that they do not recognize the https certs from Plex and you need to try by adding the plex server without plex.tv and via it’s LAN ip to confirm.