What version of CommVault are you running? I had a similar problem when I install SP 1 for Galaxy 6.1. When you install the SP it upgrade some of the security for Commvault. The problem I had was with a Windows 2000 server, Galaxy 6.1 SP1, and SQL. Here is the solution that CommVault gave me and it worked.
Symptom: SQL Backups fail after upgrading to Galaxy 6.1 Service Pack 1 on Microsoft Windows 2000
Cause: User configured for backups does not have permissions to run the backup
Resolution:
Service Pack 1 for Galaxy introduced a new Windows API call ‘LogonUser’ In order to properly call make this call, the account configured for backups must be granted the Local Security Setting “Act as part of the Operating System” right. Click start -> Run type ‘gpedit.msc’
Open the ‘Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> User Rights Assignment on the right column select ‘Act as part of the operating system’ Select ‘Add User or Group’ and add the user account of group that is configured in Galaxy for backups.