Moodle Error whenever we clicked Grade Tab under the course

We were configuring the grade setup, adding Grade Item and Grade Category, while editing halfway, the entire webpage hang. We close the webpage and login to our moodle site again, everything/every page works fine as it should be, however, whenever we click the Grade Tab under the course, it keeps hanging until the "500 Internal Server Error" page comes out.

It's only the Grade Tab that is not working now, can anyone help?