Custom ajax headers giving an "Adapter operation failed" ember-data error?

Today I'm trying to add an access token to my application's DS.RESTAdapter requests headers.

I tried the 3 solutions suggested in the official guide.

I also tried a jQuery ajaxPrefilter workaround that I found there, but nothing wants to work! :(

This one is a bit tricky to debug: my error happens during a transition, and for some reason my error template is not displayed. (I'll update this question as my debugging goes forward.)

Starting, I only faced this very obscure "Adapter operation failed" error…

Later, I discovered that the error container object had an extra errors Array field containing this single error. In my case:

{
  details: "",
  status: 0,
  title: "The backend responded with an error"
}

Yep, nice one. 8-)

Actually the things could hardly be darker at this point. I just don't know where to look for a clue anymore!

Has somebody already faced this?
Any help would be lovely!

• Ember 1.13.10
• Ember Data 1.13.15
• jQuery 1.11.3