After several days I finally got all 5 of my rokus working again. Hopefully this helps someone and perhaps roku to fix it.
The problem: I read somewhere a user report their 3810 model worked while express didn't. This person claimed the 3810 had 1 Gig of memory while express only has 512 megs. So limited memory may be why removing channels, themes, etc. may help.
Screensaver solution: Changing screensaver to a roku clock worked for some. The other screensaver that was dynamic took more memory. However, this fix did not work for me by just changing my screensaver.
My first solution worked on 3 of my devices. After changing screensavers, I had to clear the memory cache which also reboots the device. I don't know if a simple reboot would have worked. Here's how to clear the cache:
1. Goto home screen and make sure the home menu item is selected.
2. Press the Home button 5 times, then press the up arrow once, then press the rewind button twice, and then the fast forward button twice.
3. Wait until it reboots. You will probably see the menu flicker about and about 10 seconds later it will reboot. If not try again and make sure you are using rewind and fast forward as the left and right arrows don't work.
If this doesn't work, you will need to remove more items to free up more memory. After I freed up more memory, my other two devices worked again. For me, I had installed other themes previously which also installed wallpapers, screensavers, and sounds.
Initially I had deleted the theme packs but it didn't clear out the wallpapers, screensavers and sounds. To delete them just go to settings -> themes -> wallpapers - go to the My Wallpapers section, select a wallpaper and choose delete wallpaper. Repeat for sounds and screensavers.
Remember to follow the steps again to clear the cache and then see if You Tube, Prime Video, Sling, etc. work again.
