Full backup of Exchange 2010 failes socket write failed(24)

We are backing up our Exchange 2010 environment ok using a windows media server on Netbackup 7.6.0.4 . We have recently installed a Netbackup appliance and migrating all the backups to the Appliance Media server

We are however getting the following error when running the Full backup socket write failed (24) on the appliance

However a diff backup runs successfully

All Exchange are Window 2008 and the Windows firewall are switched off

All Exchange servers are on the same version 7.6.0.4