Check the logs at the client side and look for the backup command that is running, check if the group name is correct. Also, try to start the backup from the client side, using the "-g " option, to see if that works.

Can you post details of your client configuration (nsradmin output would be ideal)

Thank you for your reply.

As mentioned, manual backups (networker wise, client side) worked flawless.
After deleting ..\nsr\tmp and ..\nsr\res (not the 'servers' file) on the client side, the scheduled backup works again.

If you search forum you will find this issue has been covered in the past. Most likely your VSS backup of file system is making SQL to believe there has been another backup done in the meantime which backed up your SQL database.