Can't edit nav menu CSS anymore

Im not sure what is wrong but I can no longer edit existing menu CSS or create a new custom CSS

I can access via the little green cog / setting icon on the nav bar and I can also access the CSS and make edits but I cannot save anything. No matter how many times I click on the SAVE button nothing will committ / save.

It is driving me bonkers.

I have undertaken the following troubleshooting efforts

1) Turn off ALL plugins to see if there was a conflict. No change to issue
2) Changed browser. No change to issue
3) Clear ALL browser history from cache. No change to issue.

It is the ONLY CSS that I cannot alter. I was able to alter it at one stage but it no longer works at all. Given that I need to make some serious customisation to the navigation for this project, not being able to save changes is a real show stopper.

Any advice would be appreciated.

I have set Admin Access to ON.