Hi, this same exact thing happened to me too very recently, and it's definitely a problem on Roku's end:
-happened on both my roku streaming sticks,
-I tried all the troubleshooting stuff listed here,
-other screensavers work fine,
-I only ever had the Subaru screensaver, not the theme, but when I tried to switch back to just the screensaver after trying out other screensavers, the screensaver only option was gone,
-I tried to get the screensaver back by adding the theme, but while that got me the theme as my wallpaper, there was still only a black screen when the screensaver was supposed to kick in.
Am hoping Roku can fix this, as like the original poster, this is my favorite screensaver!