Same. My trigger seems to be the settings for refresh rate switching and resolution switching set to enabled in the new player. Disabling them on the new player allowed for playblack. It was odd since when the error msg box would display, sometimes in the background the video was actually playing fine.
Simply disabling the new player also seemed to also work, and also allowed for the refresh rate switching to work as well.
Native install of PMS on Ubuntu 18.04LTS - Version - 1.18.7.2457
FireTV App - 7.28.0.15501
