Same problem, was working fine and getting this error. Can install via xcode but through URL no longer works. Can install app using same url on any prior IOS version. If don't install via url, app still works. Something specific with IOS 14 trying to download via url

For the above issue I was finally able to get past it. Basically you have a link to a .plist file. Have to make sure your url's within that are set to https (at least I did) to get past the issue