NDMP : Error 636 read from input socket failed

Hi NBU community !

I need your help for the error in the subject. Before the explanation, i present you the context.

The environment is the next :

Master : VM 2K16 in NBU 9.0.0.1

Media : Physical server 2K16 in NBU 9.0.0.1

NDMP : Nas server  Dell EMC Unity

My company wants to backup 7 nas server in NDMP with LTO8 drives. 5 nas server are backed up really good without errors, maybe because the volumetry is low.
But, we've 2 nas servers that are complicated to back up. Each nas server does 55 To and goes on error 636 the major of time. With some chance, we can have a successful backup of these.

I checked articles about tcpkeepalive etc... but i think the problem is somewhere else. You will find in attachment the details of a failed job.

Please explain me what is wrong with my nas servers.

Thanks.