I've looked around for a solution but didn't find one... then I took matters into my own hands.
Here's the simplest way to fix it:
4 EASY STEPS
Uninstall the YouTube app from your Roku device.
Go to Settings->System -> Advance System Settings -> Network Connection Reset -> Reset Connection
Your Roku device will restart. Once it's on, reconnect your wifi, then go download the YouTube app.
Finally, after all the stress you've been through trying to cast YouTube from your phone... be amazed