A student sees “Upload failed”
The message after it says why. Usually it’s a file type the assignment doesn’t accept, or a file over 25 MB. Accepted types are listed in Assignments.
“Several files you uploaded for this assignment were not submitted”
The student has five files waiting on that assignment that were uploaded but never handed in. Each one is deleted within two days of its upload, and then they can try again.
A student handed in the wrong file
An assignment is handed in once, and a second submission is refused. There’s no button yet to reopen it, so ask the student to send you the right file another way.
A student pressed Submit on an empty assignment and nothing seemed to happen
The “please provide a text submission or upload a file” notice scrolls back into view, and is read out again, every time they press Submit. Ask them to scroll up if they can’t see it.
The admin says submission files can be opened by anyone with the address
Your web server ignores the file that protects the submissions folder. That’s normal on nginx and OpenLiteSpeed. The notice gives the one rule your host or web developer adds to the server. Once it’s in, click Check again in the notice. Until then every file has a random name, so its address can’t be guessed, and students only open files through the plugin.
The exact rules are in Protect student submission files on nginx and OpenLiteSpeed.