this may or may not apply to your issue but has solved other backup issues
check if Volume shadow copy and windows backup services are running and set to automatic in Services Console.
1. Click on Start, type Services in Start Search. When the Services page is open, scroll the page to find Volume Shadow Copy and Backup service.
2. Right click on them and check if they are started.
3. If the services are disabled, start the services and set it to Automatic and check if the issue gets resolved.
If the services are running and you get Error Message: “0x8007007B - The filename, directory name, or volume label syntax is incorrect”, file name or volume label has special characters.
The system does not accept the keyboard combination Alt+0 through Alt+32 or the following characters: \\ \\ / [ ] : | < > + ; = . ? "
This gives the system an invalid path, so please correct the file name or volume label and make sure the username you are logged into has no spaces or special characters and then try to perform the backup.
It would also be caused due to Operating System shell corruption. If your file name and volume does not have any special characters and you are getting this error message, Create a new user account and perform the backup from the new user account and let us know if it helps.