I can't create new HTML or text docs on Nightly, in any browser.
The issue comes in the 2nd screen of creating the new resource, when the document is given a name and then saved.
To recreate, choose to add a new HTML or text doc
Add some text, and press the 'continue' button.
Give the resource a name and click the 'save' button.
The page will submit, and is then displayed again. No error message is displayed.