Taxonomy term via Rest API returns 406 Not Acceptable
This works as expected using Drupal 8.3.x using latest Rest UI Mar 28, 2017.


I've done a clean Drupal install
Next created a term visiting  http://drupal.d8/admin/structure/taxonomy/manage/tags/add
Visited the term http://drupal.d8/taxonomy/term/1 to check it's there
Tested for a json response http://drupal.d8/taxonomy/term/1?_format=json
giving the expected {"message":"Not acceptable format: json"} as we have not yet configured Rest
Next enabled Rest UI module
Configured taxonomy term on http://drupal.d8/admin/config/services/rest 
Visiting http://drupal.d8/taxonomy/term/1?_format=json again gave me the expected json output like {"tid":[{"value":1}],"uuid":[{"value":"d5e87924-5ccd-4fbe-bf1b-f1790da0d800"}],"langcode":[{"value":"en"}],"vid":[{"target_id":"tags","target_type":"taxonomy_vocabulary","target_uuid":"b1d2d8a9-0f05-49e7-abbc-137aaa9d0f64"}],"name":[{"value":"Term 1"}],"description":[{"value":null,"format":null}],"weight":[{"value":0}],"parent":[],"changed":[{"value":1491899632}],"default_langcode":[{"value":true}],"path":[]} 


Note I have tested as admin and anonymous with same results. Enabling HAL and requesting the formats xml and hal_json gave the expected results too.

Disabling cookie request and enable basic_auth give a different response.


  {"message":"The used authentication method is not allowed on this route."}


Hope this helps.