After Build Page – Blank Screen Shows

Hi, I can login to my Dashboard, add a new page or post, but when I click on Publish it goes to a blank screen. Here is the URL of the blank screen page that SHOULD be my Edit Page within the Dashboard: https://www.freshfitness.ca/wp-admin/page.php

The page does build so I can see it online: https://www.freshfitness.ca/test-101/

But now I am not anywhere in my dashboard and need to hit the back button to get back to edit my page (and most of the time the changes I made to this page are lost).

I have tried disabling all my plugins. I have tried updating my theme within fresh files. I have done the automatic upgrade again but nothing seems to work.

Any advice?

Thanks in advance.