first open command prompt as administrator and run 

vssadmin list writers

Make sure no writers are in errored state

then run

vssadmin list providers see if there are any other writers other than the MS one.  If there are, you probably need to add this to the registry:

Create DWORD "nUseVSSSoftwareProvider"

Under

HKEY_LOCAL_MACHINE\SOFTWARE\CommVault Systems\Galaxy\Instance001\FileSystemAgent

DWORD Value = 1

You can also check to see if the drive you are getting the VSS snapshot is configured correctly by running

vss admin list volumes

check the filescan.log file under C:\Program Files\Commvault\ContentStore\Log Files.  Look in there for vss errors.

Finally, do the following 

From a command prompt;

diskshadow /L c:\temp\shadow.txt

list writers detailed

exit

notepad c:\temp\shadow.txt

Perform a find for "/"

You are likely to find a malformed path either a file or registry.

In my case... I found;

   - File List: Path = c:/windows\temp\hpsum\hpsum_20140505125314, Filespec = hpsum_service_x64.exe

search for the path in the registry and correct the forward slash with backslash (\)  

press F3 to search and keep doing it until the incorrect path is no longer found in the registry.

retry the backup