Suddenly for no obvious reason today I find that I cannot edit my WP site's Header section in wp-admin. I am an extreme novice with WP but I have been able to edit this section without any difficulty on numerous occasions over the past year. But now if I make any change under Appearance/Editor/Header, when I click "Update File" I receive an error page that says "400 Bad Request" "Invalid request".
I don't have a clue how to troubleshoot this. Can someone please help me figure this out. I will be most appreciative.