I am quite sure that this is a Moodle 3.8 problem.

I have debugging turned on.  When I pick a student and then pick "Complete Report" there are some "Notices" showing for every forum.  This happens for forums where students have made posts, and for forums where students have not made posts.

I do not recall these errors in Moodle 3.7, because I use the Complete Report a lot.

I am attaching a screenshot showing the Notices.