iOS app won't connect to internal local server on device

Today I noticed a strange phenomenon on iOS 11 that wasn’t there before: Quite frequently, Meteor cannot connect to its internal server anymore.

This usually happens if you start the app for the very first time after a build, and goes away once you stop the app and restart it. But sometimes, this happens during normal operation. You notice it because some images are missing.

On the console, I can see messages like this:

[Error] Failed to load resource: Could not connect to the server. http://localhost:12328/...

What may break these internal connections?!