Not the first time that something that sounds completely screwy actually works with Roku devices. Another is using a phone hotspot to connect and register a new device when it can't connect to a home network. Again, it makes no sense at all that it would make any difference, but it works.
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Can't possibly imagine how changing the theme and screensaver is going to fix an issue that is involving only Google apps when all other channels are functioning properly.
Can you imagine having less app memory available after a major OS update (additional/new functionality etc - is there ever an OS update that has a smaller memory usage footprint?), such that certain larger memory footprint apps (such as those that load external/secondary code modules like YT/YTTV, or larger complex Themes, or ad-enabled large Screensavers) cannot fully load due to the decreased memory availability (especially on low memory models w/512MB)?
If you can imagine that (and its easily imagined, since its the case), then you can understand why reducing the active memory load by using smaller-memory-using options works in certain scenarios (but not all, which is why its part of workaround solutions list).
Of course, you always have the option of not trying any of the troubleshooting steps, or just trying some of them, but considering the very low opportunity cost of resetting the Theme to the Roku default or switching to a simple Screensaver and trying to load the app(s) relative to opportunity benefit of being able to stream the app, its probably a poor decision.
There's always the default/standard troubleshooting step of removing the app, restarting, and re-installing the app (which takes longer than the other two steps combined), but if that doesnt work, and forthcoming firmware/app fixes are unknown and likely untimely, theres always the suggested workaround solutions list to reconsider.
I apologize for my ignorance. I have the exact same device on another TV with no issues, running the same theme but a different screensaver, so I never gave that any thought. I removed the memory heavy screensaver and both YT and YTTV are now loading. Thank you.
I will miss my beloved fish tank on my screen saver. However, after trying literally everything else, going back to the default Roku theme finally gave me YouTube back on my Roku Stick.
On the Insignia Roku TV, uninstalling YouTube, rebooting and re-installing (twice) finally got it back. But not even that worked on the Stick. So I'm back to the boring purple background, but able to get my YouTube exercise videos, and after 16 months of the situation life, Lord knows I need to do a wee bit more exercise.