This is an old post but might still be usable. Here are some ways to troubleshoot;
OPTION 1: Create separate object in Backup specification for the failing object affected by Volume Shadow Copy. If it is "CONFIGURATION", re-add the object manually in the backup specificaiton.
OPTION 2: Disable “Shadow Copy” option in FS backup configuration under "Options" section
OPTION 3: Restart “Volume Shadow Copy” service of the affected client and test to Enable/Disable, Stop/Start/Restart the service. Normally, setting it to "Manual" works.
OPTION 4: Modify omnirc file (client side only) and activate the parameter OB2_ENABLE_VSS_WRITERS=0 and ensure it is set to 0. Usually, in omnirc it is commented out and it set to default value of 1 which is enabled.
OPTION 5: Reboot affected client (if needed)