Some basic tip to go through parse Error json file on Odoo 11

You can get this error for many reason. When it's happening while downloading an extract of database, most of the time it's because of a reverse proxy misconfiguration.
If you don't have the time or the skills to fix it, you can go through by accessing Odoo by the IP adress instead of the url name (i.e. 48.454.548.46 instead of https://myodooerp.com).