After following the official documentation and other related posts that did not work for me, I finally found the solution on Stack Overflow! The issue persisted when trying to run the Nuxt 3 app on my Nginx instance. Pm2 would always pick port 3000, which was already in use. I needed to switch it to 3002 and finally, this is the solution:
$ npm run build $ PORT=3002 pm2 start npm -- start
May it help someone else and save some time.