Thanks. You’re right, I hadn’t rebooted.
I was prepared to send you another reproduction of the problem, but it occurred to me to try doing a check-for-update directly through the machine’s console, rather than using a NoMachine session. This check-for-update, and the subsequent update, were successful. I’m speculating that it was the running NoMachine session that was being complained about, not that a software update was running.
On the other hand, I can now check for update just fine, using a NoMachine session. So I’m not sure what’s going on, but the problem appears fixed for now.
Thanks for the help, and again I apologize for the long delays in responding. (For some reason, the forum is not sending me email when replies are posted, even though my profile settings appear to be set correctly.)