One W7/64 client can't back up to DS5100/2012R2E if the Windows directory is selected

I have a DS5100 with Server 2012 R2 Essential installed, and 13 out of my 14 Win7/64 clients back up to it successfully, every day.

But for some reason, Sylvester’s computer fails with “Backup did not succeed” and “Backup did not start on this volume”:

We can make it back up if we Customize the backup and remove the Windows directory. We attempted a “binary search” of the problem within the Windows directory, and if we select anything in the Windows directory, even a single directory (“addins”) with a single 1 KB file, then the backup will fail with the above error.

We’ve run various malware scans on this box, with no solution.

Anyone run into this problem?

run chkdsk c: /f/r as well as look for the errror in the event log scroll down to server essentials admin