Status 26 "client/server handshaking failed" during disk staging

Apologies for the late reply....

I'm battling to figure out what is happening here (plus battling with level 5 logs... too big... to much info that only Support understands).

It seems that source is P:\Lun 4 on media server  vsaus113  and that destination STU is  vsaus113_vsaul113_LTO5-DRV1 on same media server. 
Is that right?

It seems that the process is started and then simply ends with a socket error 2 hours later:

06:00:34.514 [18968.13732] <2> get_short_base: (1) cannot read (byte 1) from network: An existing connection was forcibly closed by the remote host. 
06:00:34.514 [18968.13732] <2> bpduplicate: EXIT status = 23

This looks like internal network connectivity between NBU processes on the media server is failing.

A fellow Trusted Advisor managed to solve duplication issues with these settings (followed by a reboot):

1. The paging file was set to automatic. It has been set to a fixed value 64GB (twice the physical memory) and set to use the F drive which has the most free space.
2. Increased the number of worker threads available to Windows using the following registry keys (note that the RpcXdr\Parameters\DefaultNumberofWorkerThreads needed to be created):

These recommendations (and more) can be found in:

The NetBackup Backup Planning and Performance Tuning Guide, Release 7.5 and Release 7.6  
 http://www.symantec.com/docs/DOC7449 

If this does not help, then please log a Support call with Symantec/Veritas.