The only errors that I get are due to the student progress block being installed on the home page. However this was installed way before these problems emerged.
I have checked through all of the feedback comments for odd code and found nothing other than the html written by the moodle editor.
I have discovered that our generic teacher account that has been assigned site wide password management and non editing teacher status can access the gradebook with no problems.
Gradebook
Grade book not visable to teachers
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