For something similar, I found subinacl
to be more friendly to use than powershell. Handy reference
Are you logged in a domain administrator, or just local admin on the file server?
Try logging in as whatever service account your backup process runs as ;)
Or just give yourself permission to all the files beforehand, but don’t do it manually, use subinacl - https://social.technet.microsoft.com/wiki/contents/articles/51625.subinacl-a-complete-solution-to-configure-security-permission.aspx