When I try to add a Content Block or edit an existing one in v5.3.3.1, the page reloads but just sits there as a white page.
The Javascript error console has some entries but they are nonsense. One looks like its from a JS file that is specifically not loaded in edit mode, another is a javascript error from a doctype tag ...

Any ideas on how to track this down?

I'm guessing its going to be something in my theme conflicting with the edit code?

Thanks,
James