The logs for "Windows Server Backup" are located in the "logs" folder in the Windows directory.
C:\Windows\logs
But how do you view the logs? The logs are can not just be opened and if you search on line you will find that people state that these are not to be viewed. Well that's just poppycock. Open event viewer and right click on event viewer in the tree and select open saved logs. Browse to the directory of the files and and click on them to open them in the saved logs potion of event viewer.
Open Event Viewer
Right click on event viewer in the tree
Choose to "Open saved Logs"
Browse to the file and click in it.
This should open the windows back up logs in event viewer.