Admin/manager are able to submit assignment. However, students are unable to submit assignments. After clicking "Add Submission", students will get into file submission page, but unable to submit. If a student drags a file to the uploading window, the file will get downloaded. No file will be uploaded. If the student selects the FilePicker, there are only "Recent Files" and "Wikimedia" selection, but admin has "upload a file" selection.
Moodle version: Its MoodleCloud, so its the latest version.