Exchange 2010 Backup using Netbackup & DPM in the same time

I have a new installed Exchange 2010 DAG on a two node Microsoft cluster virtual servers created on Vsphere VMware the customer has already configured DPM to take backup 

when I tried to configured the backup using Netbacku 7.5 the backup works fine when it takes the backup from one of the nodes but failes with the second

is there any possiblity that this is due to the DPM ? I mean should I tell the customer to use only one backup ? or may be the error is something else not related to the presence of DPM 

Thanks & Best Regards,