Thank you for your post.
Since this issue has not occurred previously, do you recall anything that has changed with your computing environment?
Regardless, since I have not encountered this issue previously, I have sent your post and screen shot to our Development and Testing departments for review. When I receive feedback, I will let you know.
Thank you for your reply, TSGal. It has happened ever since the two servers were set up, more than a year ago. The frequency has accelerated recently on both servers.
The Hardware Server Admin was asked if anti-virus scans were being conducted on that directory and he affirmed they were. After turning off the scans and last night's reboot after the Microsoft patch the errors have stopped.
I apologize for the bandwidth, TSGal.
Also, thanks to PhilModJunk's tangential answer to a similar question, which pointed me in the right direction.
I hope this helps someone else. I understand now that the progressive backup process is the canary in the coal mine for hardware and backup admins not applying the correct policies, and that you can't be too explicit when you are specifying FileMaker Server directory exemptions from admin processes.