Sorry for the lack of any response here - last year I had a kid and the time I have available for working on this app is hovering somewhere below 0 hours. I'd definitely make an effort to carve out time if anyone has a pull request for me on the open source BitBucket repo (https://bitbucket.org/alecgdouglas/gbenthusiast) but beyond that, I just can't.
@flammabled has the workaround for the 404 issue - you need to clear app data. You'll also probably have to refresh the video feed (bottom row, "Actions" -> "Refresh Videos") a few times in order to fetch the older videos as well.
Unfortunately, because progress syncing hasn't been implemented, clearing app data will also wipe out your progress on videos so you'll want to make note of where you are in videos before doing this.
I haven't had time to look into the 404 issue beyond just verifying that it fixed it for me too, but my guess is that the cached URL for older videos saved in the app is no longer valid after some migration occurred on Giant Bomb's end.
This is a side-effect of a more fundamental problem with how the app fetches and manages data from the GB API, and is also related to how you need to refresh the videos a few times before you get older videos showing up at all.
I'm actually looking at that issue - figured I would try to get the progress syncing working, and then do the local storage stuff if I still had time after that. No promises - I saw that someone else made an attempt a while back - but I do have a branch locally that updates the leanback to the latest version, updates to SDK 28, fixes some layouts that broke when I did that, some initial work on adding those API calls, the easy stuff. But I'll post again if I get something working.