Theme Composer page doesn't load - Ver 9.5.4

Walkthrough of issue in vid below.

Quick update that I tried editing the theme in Incognito Mode in Chrome, as suggested by Amy Dewaal here:

The Incognito Mode trick worked once! Upon creating a new theme, the page loaded properly. But as soon as I refreshed the page, the new theme began hanging again.
(also, going into Incognito Mode just to edit a theme seems a little silly. :p)