Unable to view pages in Cornerstone: console warning: Frame missing CORNERSTONE_FRAME signature (cont...)

I posted this issue not long ago and still have not been able to resolve it. I have followed all of the steps provided:

  • clear server and object cache
  • remove PRO theme
  • install fresh PRO theme
  • clear all browser cache
  • clear Cornerstone cache

The issue has persisted. I have deactivated all plugins as well as all custom hooks/code.

What is the next step in diagnosing this issue?