500 Internal Server Error on docker-composed Flask app using Postgres

Successfully deployed my Flask application together with Postgres image using docker-compose.yml, no errors during the process. The site even somewhat functioning apart from index home page and form submissions (e.g. http://localhost:4000/login, http://localhost:4000/register pages pop up in the browser). The home page displays 5000 Internal Server Error...

I have connected to the postgres docker images via psql and checked if the database with the right name has been created, and yes - it's there in the list. So I'm having hard times figuring out what might be wrong..

Ah, and yes, here,s my GitHub repo by the way: https://github.com/irmscher9/npdaw