Why do I get "Type does not correspond to an entity on this site" when POSTing to REST api behind an Apache Reverse proxy?
The solution is to remember set the base url of any _links href values to use http if Drupal expects it, and https if Drupal, instead, expects that.

In my case, I was using https, but the https connection was terminated at the Apache reverse proxy, so Drupal wanted http.

Using a REST extension to test with let me manually change the links array to reflect that, and the user was created just fine.

Also, the "type" href should not have the _format=hal_json on it.