When getting VSS Volume Shadow Copy Service error , it's usually due to NTFS drive not having enough space , VSS core services alone requires at least 500 mb for each volume to be shadow copied , Try to check the event viewer and look for VSS logs , Once you made sure there's enough space on disk and you're still getting the error , You can try to use Regsrv32 util tool to register and un-register vss OLE Controls and Dlls in windows registry . 

Try to create the following .bat and execute :

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

backups usually starts to run afterwards , If the issue still persist (possibly in windows 2k8) it might be due to bareos picking wrong mountpoint while taking snapshots  try checking the configs .