The Windows Backup 0x80070020 error is a sharing violation error, there is a file or files that have open handles to another process or processes that are locking the file or files from being copied by backup.
someone I helped with this error had the issue with his C:\Users\<Username>\AppData\Roaming\Microsoft\Windows\Cookies\index.dat file.
I had to use Process Monitor to find out which file was actually locked. The only way I managed to resolve it was to create a selective backup to exclude the C:\Users\<Username>\AppData\Roaming\Microsoft\Windows\Cookies folder with that persons permission.
Just a note that it might not be that file that is locked, it could be any other file being backed up.
The Process Monitor filters I used were:
Command Line contains C:\windows\system32\svchost.exe -k SDRSVC
Result is SHARING VIOLATION
You can download Process Monitor fromhere.