Asking for enrollment key when submitting

This issue is popping up intermittently. A couple teachers have encountered it and a few of my own students have just come across it.

There is an online text assignment that they fill out. When they go to submit it, they get a page asking for the enrollment key. These are students that were already logged in and enrolled in my course. When they enter the key, it takes them back to the assignment where their work is gone.

It's frustrating.

We are Moodle 2.3.1+ from 07/12