backup sharepoint with backup.exec 12.5

Hello,

I try to backup my sharepoint server with backup.exec 12.5
I have agent to sharepoint and I add a server farm, but when i try to connect to the new server farm i have a error message:

"an error was encountered whie attempting to browse the contents of farm0_moss0.
although the specified server is a sharepoint resorces, backupexec cannot confirm this topology.
review your server credentials and ensure that the services are runnig on the web server".

Is someone can help me to understand this error, please ?

Thanks,
tali