I've started to get this error lately with Boards manager:
Error downloading https://downloads.arduino.cc/packages/package_index.json
Any clues how I can fit it?
Could be one of many things.
Re-start your router. (based on personal experience as the routers cache was corrupted)
See if anything changed in your computers security / antivirus settings and allow the IDE through any firewalls. (common issue)
Have you recently changed the IDE in any way eg. folder re-directions etc.
Just guesses based on minimal info supplied.