I would try Restarting the Volume snapshot service. If that does not work for you. the next step would be:
1. Recreate the backup set.
2. On The options Screen, choose the option for Incremental \ Differential backups.
3. Below that choose the option for "full backup"
What this will do is instruct UltraBac to use its own driver to freeze the drive.
Test Engineer / Beta Manager