I'm not sure that excluding a volume shadow copy is actually what you need to do. I thought I was in the same boat as you... Windows Backup and Restore was reporting failed backups due to malware. The only references I could find in Windows Defender was to a path similar to
Device\HarddiskVolumeShadowCopy5\Download\something.crx, and searching similar paths on my actual drives wasn't turning anything up.
My first clue was when I tried to redo the backup manually, I noticed the first step was "Create Shadow Volume." This made me think that Defender must not be so stupid after all, and perhaps it was catching something being copied from a source drive. After futher investigation, turns out some symbolic links (folder aliases) I had created was confusing the issue and I finally did turn up the reported file (downloaded over 5 years ago!) that it was complaining about. Now why full scans from Defender doesn't find it, but real time access during backup does, is a separate issue.
Likely you aren't as inept as me with locating the reported malware file(s), but maybe you do have a tenacious bad guy that is either having trouble being cleaned up, yet hiding itself well, or that keeps re-infecting the system from another vector.