If VSS snapshots are failing via Bacula please check the following:

  • There is at least 5% of disk space available on system drive
  • the VSS plugin is installed
    To check this, look for Bacula VSS Plugin under Control Panel > Add/Remove Programs

If the above is OK you may need to re-register the VSS services and associated files via a batch file.
 
Open Notepad on the client server and paste the following:
 
cd /d %windir%\system32
net stop vss
net stop swprv
regsvr32 /s ole32.dll
regsvr32 /s oleaut32.dll
regsvr32 /s vss_ps.dll
vssvc /register
regsvr32 /s /i swprv.dll
regsvr32 /s /i eventcls.dll
regsvr32 /s es.dll
regsvr32 /s stdprov.dll
regsvr32 /s vssui.dll
regsvr32 /s msxml.dll
regsvr32 /s msxml3.dll
regsvr32 /s msxml4.dll
vssvc /register
net start swprv
net start vss
 
Save the above as vssregister.bat to the desktop of the client server.  Run the batch file by double-clicking.  This should run and then close upon success.  
Open a command prompt and type/paste the following:

vssadmin list providers
 
There should be at least one (typically Microsoft)

You can also download the batch file as attached to this article:

vssregister.bat

Run a backup to make sure everything runs correctly and the backup completes.
If you still have failures, look in event logs and make sure there is enough available unpaged RAM.

You may see an item like:
The shadow copy of volume C: could not be created due to insufficient non-paged memory pool for a bitmap structure.

You can also check if a VSS/snapshot will work on the server in question along with the drive in question by opening a command prompt and pasting the following where "D:" is the drive you want to check:

vssadmin create shadow /For=D:

Here is an example of drives C: and D: being successful :

 

Here is example of C: being good and D: failing: