Hi
It’s difficult to say for sure without proper error messages and stack traces, but there might be different causes of some of the crashes in this thread. e.g. I doubt Guided Access has anything to do with the inability to download assets.
I have seen crashes on Android where the error message is “Unexpected end of script” and I believe that’s related to incomplete downloads of the JS bundle which happens by default when the app first starts. If you’re seeing “Could not download …” for assets then maybe your users are also sometimes getting partially downloaded JS bundles. When the app starts next it will try to run the code and find it truncated, causing a crash and an “Unexpected end of script” error.
Here’s one of my posts about this on Android with a workaround that seems to have fixed it for me (although I don’t have an app on the App Store/Play Store, and hardly any users, so I can’t say for sure):