VAST tag url error: Ad error: AdError 1012: Unable to request ads from server due to network error

I have created a VAST xml version 2 which follows the proper standard. I am trying to validate it in vast tag validator

There are two options in the validator. Either to upload VAST tag or VAST XML. I paste the VAST tag (i.e. the link to the xml file) it gives me this error

Ad error: AdError 1012: Unable to request ads from server due to network error.

However, when I paste the xml itself into the VAST xml field option, the ad runs perfectly fine. I cannot share the xml because of security reasons. Is there a way to tell what possibly could be the problem?

I reproduced (10:51:46) Ad error: AdError 1012: Unable to request ads from server due to network error. issue.

After review of console errors I figured out the issue was related to CORS.

Then I added https://imasdk.googleapis.com to allowed origins list of my API and the issue was solved. Hope this is the root cause for you as well