Symptom Cause / fix
Browser/curl rejects the cert Server is on ACME staging (untrusted). Switch off staging + restart, or use curl -k for testing. (Switching staging→production now moves the cached staging cert aside automatically so production re-issues.)
New domain shows a cert warning at first, but works in an incognito window (then your normal profile "catches up") A browser reached the URL before issuance/DNS settled and cached the failure. The client now verifies a browser-trusted certificate is actually being served before it prints Tunnel live — wait for that line, then hard-reload the tab (Cmd/Ctrl-Shift-R) to clear the cached error.
404 no tunnel configured No frontend exists for that Host. Create one (serve).
502 tunnel offline The frontend exists but no client is connected. Start the client; it auto-reconnects.
403 Blocked host (Rails/Django/etc.) Your app rejects the public hostname. Add it to the framework's allowed-hosts (e.g. Rails config.hosts).
First request to a new domain is slow / fails Cert is issuing on demand; ensure DNS points at the server and :80/:443 are reachable.
Too many requests (429) Repeated bad API keys/tokens trigger a temporary per-IP lockout. Wait, then use the correct credential.