Can you please give me the steps/links clicked in order for me to reproduce the Customizer not loading?
I logged in and clicked Customizer under the Appearance WP menu item and the Customizer loaded just fine.
I exited the Customizer and then navigated to Connections : Templates admin page and clicked the Customize button for both cMap and the Default template. In both instances the Customizer loaded fine.
I looked at the recently active plugins list and reactivated the Child Theme Configurator plugin and repeated those steps. The Customizer still loaded as it should. I then deactivated Child Theme Configurator since it did not seem to make a difference.
I’m guess I am misunderstanding what the issue is exactly so the have the exact steps to reproduce will help me replicate and debug it.