Whenever you click on the Back up now
button, it creates an incremental backup of what was included when you first setup the backup. Since that file's path was included when you setup the backup, that would be why you are getting that error log message since it had been deleted and cannot be found to back it up now.
You will just need to click on the Change settings
link in "Backup and Restore" to change what is included in the backup, and make sure that the deleted file's path is no longer included. Using step 4 in the tutorial below can help show you more details on this. Backup User and System Files
Hope this helps,