When you install the Woocommerce Square plugin, GP Premium is broken. Specifically, when you try to operate Appearance->Customize, you get a white screen with the message “There has been a critical error on your website. Please check your site admin email inbox for instructions.”
Of course, no email arrives, so there’s no explanation of what to do to fix the problem.
Deactivating Woocommerce Square plugin fixes the problem. Activating it breaks things again.
I’ve already tried removing the Woocommerce Square plugin and reinstalling it – same problem.