Verify that the Vault Service Account is an admin on the Isilon and has full rights to the folder target.
Read KB2686098 and apply one of the two workarounds suggested by Microsoft.
Could not read the volume information for \\isilon.network.local\archivingpath
Reason: Invalid Signature
Check that the File Server is available and that you have the correct permissions to access the device.
A Dtrace of MMC and AdminService does not reveal this error message, as it is occurring at the Isilon OS level.