In my Course I have multiple Assignments. When viewing one of the Assignments for grading and clicking on the "View all submissions", a list a submissions are shown in the /mod/assign/view.php page. When I click on the Grade button, the view page gives a 403 Forbidden error.
I had same error when trying to view user profiles, but this is now working after ModSecurity was disabled on this site. My host provider looked into this issue and replied "There is no error logged in web server"
This same /mod/assign/view.php page works fine when you click on the "Grade" button next to "View all sunmissions". But when 'rownum' is passed as a parameter it goes to 403 Forbidden.