Just to let you know.
Literally minutes after I posted my comment to you I got a very similar error message to your own regarding my own Backup. Talk about coincidence.
I took my own advice, and opened File Explorer, then right-clicked on my back-up drive and selected to Scan it for infections. No infections there.
So then I cleared my browser caches, and as an extra precaution, ran the Disk Cleanup app that comes with Windows 10 (just type Disk Cleanup in your search box).
After doing those three things I went to backup and started it again. Worked fine with no alerts about virus.
The tools Norton mentioned are fairly popular. Ccleaner is good at digging out all the caches in the %userprofile%'s %localappdata% directories. I prefer to do this manually myself, but the tool definitely works and is free too, so worth trying.
The other two tools do similar things: scanning for malware on your system.
I'm not a fan of SuperAntiSpyware though. While there are settings you can adjust, for the novice user it can be rather aggressive in pointing out cookies that it suggests removing, which in fact are just session cookies that save you the hassle of re-signing into websites or your online email etcetera. But that is just my opinion - again the tool is free and most certainly works - so if you feel like trying it don't let me persuade you. If you do not like it you can always uninstall it.