Using postman to put (update) data on a heroku postgres, returns 500

System Information
  • Strapi Version: 3.6.8
  • Operating System: MacOS Catalina
  • Database: postgres
  • Node Version: 14.8.3
  • NPM Version: 8,3
  • Yarn Version: 1.22.17

Hi! I want to use postman to update a JSON field on a heroku postgres installation. Locally everything works fine. GET (from heroku) works fine, too. The proper access rights have been set in Settings => Roles (update is checked), auth is not required.

This is what is returned:

{"statusCode":500,"error":"Internal Server Error","message":"An internal server error occurred"}

Do I need specific headers to make put work?
Or what is my mistake?