Same here. I purchased 3 Roku Streaming devices and loved the default fish screen saver. Now after no activity it no longer shows a screen saver just a black. I've tried changing the screen saver and restarting the Roku devices and still nothing.
While the OS 10.5 update seems to have fixed many things, I'm also seeing screensaver issues. I have two Streaming Stick+ (3810x) devices. Both have been updated to build 4201-50. All theme category settings are identical. The screensaver on one activates properly, the other never activates.