"Access Denied" when trying to edit Responsive HTML5 Publish Settings
I am using FM2019 on Windows 10.
From FM, I am publishing to Responsive HTML5 in the FILE > PUBLISH menu.

When I try to save / save as / save and close the settings, I get an "Access Denied" message.

I have admin rights on the machine and nothing has changed (so far as I know) between the last time I could edit the file and now that I cannot.
Any idea what is causing this and how I can fix it?
