spurious kernel panic message on restart

I upgraded my husband's MacBook to High Sierra. All went well. Now, however, when I shut down normally and restart, it's coming up with the message "You shut down your computer because of a problem."

But that is not the case. The shut down was normal. In fact, the last couple of times (because I was trying to figure this out) I ensured that no app was active at shut down.

I found this message described here:

https://support.apple.com/en-us/HT200553

Other than the discussion of this message display, the other aspects do not apply (no kernel panic screen had been displayed, no crash report screens or details, etc.).

How can I clear this message so that the restart proceeds smoothly?