Ultra 4800, OS 9.4.1 build 4236. Went to check for updates, and Plex was updated. The completion message appeared on the screen, then the Ultra froze. It would not respond to any button presses on the remote. After about 30 seconds, the screen went blank, then the Ultra rebooted. It appears to be working properly.
I generated a issue ID: W9-087-749.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.