I'm able to repeat this so I'm fairly certain what the problem is and how to fix it. My Roku 3/2 seems to have no space whenever I back out of an ad or ad set right in the middle of them. If I do that and then try to do Netflix or YouTube they have no space launch issues. When that happens I go to any Movie Channel with ads like Plex or TUBI, then start a movie. After I watch the first set of ads and the movie has resumed, I go back to the channel that wouldn't open and now it's fixed.
When you finish watching those ads it clears out all the cache and you're good to go.