Strapi on Heroku - issue with the JWT token

System Information
  • Strapi Version: 4.2.0
  • Operating System:
  • Database: Postgres
  • Node Version: 14.18.1
  • NPM Version: 8.13.0
  • Yarn Version:

I have a local, vanilla Strapi installation deployed on Heroku using Heroku CLI. Admin UI and Content API work fine but I cannot log in frontend users. All I can get from the /api/auth/local endpoint is

    "data" : null,
    "error" : {
        "status" : 403,
        "name" : "ForbiddenError",
        "message" : "Forbidden",
        "details" : {}

I can get JWT from the local installation at http://localhost:1337/api/auth/local and have no problem with the same installation and environment on the DigitalOcean App service. Just Heroku is giving me a headache.
I’m using curl for checking JWT:
curl -d "identifier=********&password=*****" -X POST https://****** Again, same line works fine on localhost and on DigitalOcean App service.

Any suggestion would be greatly appreciated.