As I mentioned in an earlier post, I had a similar issue running on both Windows 7 and Windows 10.  This led me to think that this may not be a "QuickBooks" issue, but rather a Windows user issue instead.

I opened Control Panel -->Administrative Tools > Task Scheduler and looked at the actual task within Windows.  I had the user set to the user that the task ran under (local user with admin rights to the local computer).  

I right clicked on the task and selected Properties.  In the General Tab, in the Security Options section, I changed the When running the task, use the following user account from the local computer user name to Administrator as well as checked the Run with highest privileges.

Doing that allowed the backups to start working again (successful backups for a week)

I hope this helps.