I do not believe that debug extensions have anything to do with your GUI problem. These extensions are there to help with OpenGL debugging and would not be enabled in release builds but only during development and debugging. (https://www.opengl.org/registry/specs/ARB/debug_output.txt)
I would printf or UE_LOG a bunch of statements around the places where your GUI does not display and see where that gets you. It’s possible that something else is causing the GUI not to appear…