In production, the error is cannot get /. Why?

Question

Express.js

I am using nuxt with express server. When developing in development mode, everything works like clockwork. But as soon as I do a beat (npm run build) and then run the application (npm start) something strange happens. The application starts, connects to the mongogb database, but when you go to localhost: 3000, you get a cannot GET / error. What could be the problem? There are no errors at all. In nuxt.config, I set the path to the server's root file in the serverMiddleware field (the path works because it connects to the database).