When a student submit an assignment (on a page /mod/assign/view.php?id=60206&action=editsubmission), after clicking Save button, it redirects to page /mod/assign/view.php and show 500 error. After enabling debugging, the page /mod/assign/view.php showed error as below:
Fatal error: Cannot declare class mod_assign\event\course_module_viewed, because the name is already in use in /var/www/moodle/mod/assign/classes/event/course_module_viewed.php on line 0
Even after purging all caches, the problem still exits. There was no such problem before. I don't konw why.