Update: the solution that I mentioned does not really solve the problem. After doing this on a site, the problem went away and then returned.
I'm wondering if this has anything to do with your maintenance tonight Mark.
For the moment, I am proceeding with my own maintenance and will get backups one way or another, but if I find that this problem is still occurring in the morning, I will have to shut off WordFence until this is resolved.
Thanks!
Art