Sharepoint - Unable to initialize VSS requestor

Hi, we are trying to backup Sharepoint through the VSS plugin, but I'm getting this error:

2013-09-13 14:48:50 avmossvss Warning <14104>: No SharePoint installation was detected on the system.

2013-09-13 14:48:50 avmossvss Info <13955>: Initializing VSS requestor...

2013-09-13 14:48:50 avmossvss Warning <16253>: No SharePoint VSS Writer instance detected - terminating initialization

2013-09-13 14:48:50 avmossvss Error <0000>: Unable to initialize VSS requestor! No SharePoint installation was detected on the system.

"vssadmin list writers" show no problem.

Any ideas ?