Rbackup, when installed as a Windows Service, logs in using the default SYSTEM Account. There is a Server service registry parameter that enables tighter security by allowing you to specify by name which shares (and pipes) the system account can access.

Important Note: Using the Registry Editor incorrectly can cause serious problems that require reinstalling the operating system

THIS REGISTRY KEY SHOULD BE CREATED ON ALL THE REMOTE MACHINES THAT ARE BEING MAPPED, NOT ON THE MACHINE WHERE RBACKUP IS RUNNING.

1. From the Start menu, click Run

2. In the Open box, type Regedit (will open the Windows Registry Editor)

3. Navigate to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LanmanServer\Parameters

4. Check if you have RestrictNullSessAccess key in the path. If not, you will have to create one.

5. From the Edit menu, select NEW, DWORD Value.

6. This will create a New Key on the right-side panel of the Windows Registry Editor. right click on the New Key and select Rename.

7. Type in the Name of the Key as RestrictNullSessAccess (Case Sensitive).

8. Now, right click on the RestrictNullSessAccess key and select Modify. In "Value Data" enter 0 (Zero) and click OK.

9. Close the Windows Registry Editor, restart your computer.

Also read through this Windows Article, which explains how to create this key.
http://support.microsoft.com/default.aspx?scid=kb;EN-US;122702

You can also specify lists of share names and pipe names that can be accessed by the system account, which is explained in the following link.

http://support.microsoft.com/default.aspx?kbid=289655#Task1