Robocopy fails with security error copying "from nas to nas". Why?

[Hi all. Sorry for cross-posting. I originally posted this on stackoverflow, but later realize it's a better fit for server-fault. ]

tl;dr

robocopy has security problems copying from 'nas to nas'

The system detected a possible attempt to compromise security. Please ensure that you can contact the server that authenticated you.

Summary

I'm running into "windows permission problems" when making backups using using the following:

• powershell
• robocopy
• Windows 2008R2
• Windows task scheduler

Task Scheduler output

Taskscheduler runs under user domain account "OPS\backupuser"

The script succeeds when it copies "from local drive" "to the backup nas"

However it fails when the script copies "from another nas" "to the backup nas"

In pictures...

Success: local drive --copy-to--> backup NAS
Fails:   another NAS --copy-to--> backup NAS

Output

Robocopy fails with exit code 16.

Here is detailed output:

-------------------------------------------------------------------------------
   ROBOCOPY     ::     Robust File Copy for Windows                              
-------------------------------------------------------------------------------

  Started : Thu Jul 07 22:22:11 2016

2016/07/07 22:22:26 ERROR 1265 (0x000004F1) Getting File System Type of Source \\app-data-nas.hosting.acme\bazapp$\production\foo_industries_prod\
The system detected a possible attempt to compromise security. Please ensure that you can contact the server that authenticated you.

   Source - \\app-data-nas.hosting.acme\bazapp$\production\foo_industries_prod\
     Dest : \\dr-backup-nas\AppDR$\ALL_DR\FOO_INDUSTRIES_DR\foo_industries_prod\

    Files : *.*

  Options : *.* /NDL /S /E /COPY:DT /PURGE /MIR /B /NP /R:0 /W:1 

------------------------------------------------------------------------------

Other points

1) Because I the environment is 'locked down', I could not run this from the command line, either as :

• my own account
• my own account with elevated command prompt
• OPS\backupuser

2) I tried adding '/NODCOPY' , but robocopy failed; apparently we don't have the hotfix for this option.

Thanks in advance!