So you've switched some website ad inventory/ property/ tag to using a "responsive ad" unit tag/format? And then you head over to the website and check if ads are served correctly. And the ad slot remains blank? No ads are serving? You already checked the Google console and it's doesn't show anything (helpful or at-all)? Then you open Firebug or Chrome's web tools and the Networking tab shows that the JS triggered requests to Google's AdSense server fail with error 400 Bad Request?
Do this: Be patient. And wait. For an hour or so.
As it seems, especially with responsive ad tags, Google is slow in registering newly created ad tags/units into their system. And until Google is done registering the new ad unit, Google's servers return a HTTP 400 Bad Request error response code, and the ad slot remains empty.