As it’s said, once you solve problem #1, problem #2 gets a promotion. So, after fixing the problem with downloading package_index.json file, Arduino IDE immediately started complaining about file integrity.
To be honest, I’m not sure why this mess has happened but this is a really annoying thing. Fortunately, the problem can be solved easily if you have correct files.
So, let’s get back to the problem. The problem happens when you open the board manager in Arduino IDE.