If you've read my previous posting...turns out (at least my testing so far indicates) that removing "Separate groups" makes the errors go away, but it also removes a student's ability to see others' work.
What is the intent of the paring of "Individual wiki" and "Separate/Visible" groups? The teacher I'm working with assumed that that was the correct setting for the above intent. SInce the errors I'm getting are intermittent (and the visibility between students also seems to vary from wiki to wiki), I'm trying to suss out what's supposed to happen "under the hood," so I can strike a compromise, or contribute to the bug tracking.
It looks like almost everybody else interested in this thread was using "Collaborative wiki" with "Separate/Visible Groups," which there seems to be workarounds for. Moodle docs is zero help.