Drupal 8.5 REST 406 Not Acceptable I had a REST resource for a custom entity working in 8.3, but when I try hitting it in 8.5, I get a 406 with message": "Not acceptable format: json" in the body.

Here's my config, with json clearly defined as the format:

langcode: en
status: true
dependencies:
  module:
    - basic_auth
    - MY_MODULE
    - serialization
id: entity.MY_ENTITY
plugin_id: 'entity:MY_ENTITY'
granularity: resource
configuration:
  methods:
    - GET
  formats:
    - json
  authentication:
    - basic_auth

I've tried stepping through code with Xdebug in PHPStorm, but not having much luck seeing where this is coming from.
You have to add this on the end of your endpoint URL:

?_format=FORMAT. So if you want JSON, its ?_format=json.

This is a new change in 8.5.0 requiring this argument, but I could not locate in the changelogs exactly why it is now required otherwise a 406 is returned.

This same thing happened to all my REST endpoints until I added that.