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If one or more of your Students has been automatically unenrolled from your course, here's what to do:
Look under Participants and you'll see that in the right column of some enrollees, there's may be enrollment expiry date set.
(This means that after a particular date, the student will become unenrolled.)
In the box on the right where you see that text, click the little "gear" icon.
in the screen that appears, disable the "Enrollment ends" checkbox. That fixes the issue for your current students.
Repeat this for any similarly affect Student in this or any other of your Moodle courses.
Also, check the Enrollment Methods in your course:
You should ensure that the Self Enrollment Method itself doesn't have these kinds of enrollment duration restrictions either. Again, in Settings >> Users, select Enrollment Method, and then click the little gear icon next to Self-enrollment. On the screen that appears, look at Enrollment Duration (which I saw set to "35 days"), and "End Date", farther down the screen. These would affect how long any self-enrolled students could access your course (e.g. the ones who use your enrollment key to self-enroll).