see Unable to open Anki
and json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0) - #11 by Eldana
After using the latest version, if you wish to open your collection with an earlier Anki release, please go to the File>Switch Profile menu item, and click on “Downgrade & Quit”. If you skip this step, you may get an error message when opening your collection in an older Anki version, and you will need to return to this version, downgrade, then try again.