Drupal 8.5 REST 406 Not Acceptable
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.