Why does Chrome keep displaying "Site cannot be installed: the page is not served from a secure origin" in the console?

Question

Whenever I look in Chrome's console, I see this error message:

Site cannot be installed: the page is not served from a secure origin

This only started happening a few days ago, and Chrome wasn't updated in the meantime.

Answer 1

2019-Dec update: this answer is outdated. Neither of the two flags mentioned in it are available in Chrome any more. Old answer below.

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This error is caused by sites served over HTTP if "Add to shelf" is enabled in chrome://flags.

Solution: go to chrome://flags/#enable-app-banners (used to be chrome://flags/#enable-add-to-shelf) and change "Enabled" to "Default", then relaunch Chrome.