GoogleIMA: 1009 The VAST response document is empty

THE SITUATION:

I am trying to display preroll ads videos on my app.

I have two different adTagUrl.
One works fine and the preroll ad is displayed.
The other doesn't, and I get an empty VAST response instead.

I thought there may be something wrong in the code, so I recreate a basic small app taking the code from the google IMA repo example.

The results is the same. One work and the other doesn't.

THE ERROR:

errorCode: 1009 errorMessage: "The VAST response document is empty." type: "adLoadError" 

VAST INSPECTOR:

If I check the VAST ad response using the Google Video Suite Inspector, both the adTagUrl work.

THE CODE:

The code is very straightforward, taken from the google IMA example repo:

var adsRequest = new google.ima.AdsRequest(); adsRequest.adTagUrl = 'MY_TAG_URL'; adsRequest.linearAdSlotWidth = 640; adsRequest.linearAdSlotHeight = 400; adsRequest.nonLinearAdSlotWidth = 640; adsRequest.nonLinearAdSlotHeight = 150; adsLoader.requestAds(adsRequest); 

THE QUESTION:

Why am I getting that error?

How it's possible that the adTagUrl works fine in the Vast Inspector, but it doesn't work when I request it from the code?