Ribbon does not display when customising Default Form Web Parts

I have recently begun experiencing an issue where the ribbon does not appear when accessing the Customise List > Form Web Parts > Default New Form (or any other form types). I am able to select Default New Form and it loads the page to edit, but the ribbon for that page does not display, as such I can not edit the web parts or form layout. 

Inspecting the ribbon element in the browser shows that it received a 403 Forbidden error and did not load the ribbon. I have Full Control permission of the list I am editing, and am the Site Collection Administrator so not sure why it's generating a 403.

Is there any solution that might address why this is occurring? Currently this means we cannot customise any new/edit/display forms.