Code
T0017 wrong-name
A request named a host that is none of the app's own names. A caller gets it with the status 421.
Details
An app knows its own names. With no address it answers only as this computer: localhost, 127.0.0.1 and [::1], on the port of the engine. With names of its own, address of engine.open, it answers on those, each with its port where that is not the usual one. A request that names any other host is refused before anything else is looked at. So a page of another site that points a name of its own at this computer cannot reach an app that is being developed.
address: '*' answers every name: it is for an engine behind a proxy that itself lets only the right names through.
The fix
Reach the app by a name it was given. On a server, give the app its names when it is opened.