There have been several threads about this which all end with someone from Roku support saying "uninstall and reinstall the app." Which works, temporarily, but I've found it to be unnecessary: if your video freezes after you skip or finish an ad, click up on your remote to show the youtube video options. Change the streaming quality to one notch lower than it currently is. Wait for a few seconds, and the video will automatically reload at the slightly-lower resolution. This has always resolved the issue for me.
I think what's happening is that the youtube app might be requesting a slightly higher resolution + framerate than the roku device itself is capable of handling, or is otherwise having trouble with in YouTube's particular format. When in doubt, 1080p or at worst 720p are pretty sure bets.
Anyway, give that a try, see if it works for yours. I've solved the problem on all three of my Roku TVs and my Roku Express this way.
It's still a band-aid and I'd rather it just be fixed outright, but with Roku and YouTube bickering with each other like they are currently, I'm not holding my breath.
