Troubleshooting

Find what you're seeing in the first column. Most "my app won't open" problems are one of the first three.

What you see → what it means → what to do

What you see	What it means	What to do
"Nothing on port N" page	The address is right, but no app answered — it isn't running, or it's still in private mode (localhost only).	Start the app, and start it on 0.0.0.0. See step 1.
Blank page / "invalid host" / a 400 error	The app didn't recognise the web address it was opened with.	Add your address to the app's allowed list (Vite allowedHosts, Django ALLOWED_HOSTS, CRA host-check). See step 3.
A login or form fails with a 403 (Django)	Django needs to trust your address for secure form submits.	Add CSRF_TRUSTED_ORIGINS = ["https://<your-address>"].
"Mixed content" / it jumps to http:// / broken localhost links	The app doesn't know it's being opened securely, or the page has http://localhost hard-coded in it.	Tell the app to trust the gateway (ProxyFix / trust proxy / SECURE_PROXY_SSL_HEADER); use plain /... paths in the front-end.
Live-reload doesn't work; errors mention ws://localhost	The auto-refresh is trying to reach localhost.	Point it at the secure address on port 443 (Vite hmr.clientPort: 443, CRA WDS_SOCKET_PORT=443).
It sends you to your lab home page	The name in the address isn't a known shortcut or a 2000–9000 port.	Use a valid name (app, svc, …) or a port in range. See the addresses.
An app on a port like 1234 or 9100 won't open	That port is outside the range the gateway routes.	Move the app to a port between 2000 and 9000.
An http://… link	Normal — it switches you to https:// by itself.	Use https://; nothing to fix.
The browser can't reach the address at all (can't find it / times out)	Wrong lab number, the computer is off, or a work VPN/firewall is blocking it.	Re-check your lab number; try a phone hotspot to rule out a work network.

The one check worth running

On the lab computer (in code-server's terminal, Jupyter, or SSH), see what your app is actually listening on:

ss -tlnp | grep :5000      # use your port

• 0.0.0.0:5000 → good; the gateway can reach it.
• 127.0.0.1:5000 → this is the problem. Start it on 0.0.0.0 instead.
• no output → the app isn't running (or it's on a different port).

Still stuck

• Re-read Run your own app for your tool.
• Paste what you're seeing into an AI assistant using the diagnostic prompt.
• Ask your instructor — include the exact address you opened, the message you saw, and the output of the ss command above.