Container crashes immediately → check docker logs romm. If it's invalid host in "tcp://..." you're on Kubernetes, see Kubernetes Troubleshooting.
Database connection errors → verify DB_HOST/DB_PASSWD match your DB container, and that the DB has finished initialising (first run takes longer than you'd think).
"Page not found" on first load → wait, because initial migrations and resource seeding take a minute.
No Setup Wizard
If you don't see the setup wizard when first connecting, and instead see a login screen, check if the version number in the bottom right is 0.0.0. If it is, RomM is functional and accessible, but not configured correctly, and you will not be able to login, no matter what username and password you try.
Ensure that you have modified path/to/library,path/to/assets, and path/to/configs in your docker-compose.yml so that they point to directories on the host.
/romm/library, /romm/assets, and /romm/config should remain unchanged, as these refer to directories inside the docker container itself.
EmulatorJS won't load/404 → In-Browser Play Troubleshooting (on the slim image, cores come from a CDN at runtime, so a 404 usually means outbound networks are blocked).