Why does Robocopy gives ERROR 5 (0x00000005) Getting File System Type of Source but still copies everything?

I am trying to run a basic Robocopy script to mirror one folder and all its contents from one server to another but getting ERROR 5 (0x00000005) Getting File System Type of Source. However, the copy process appears to be successful according to the Robocopy log and what I see in the destination.

Can't figure out what is causing this. Even though everything appears to be successful this error causes scheduled task job to report a failure. I have read/write access on both source and destination and verified by opening the folder, creating a new folder, creating a file then deleting both.

Source is a Microsoft Windows 2012 IIS Server Destination is some sort of Unix web server. Not sure exactly what but I can access it using UNC path just like any other windows network share.

Robocopy command I am running is:

set dt=%DATE:~10,4%-%DATE:~4,2%-%DATE:~7,2%_%TIME:~0,2%.%TIME:~3,2%
set dt=%dt: =0%

robocopy /MIR /COPY:DT /XA:SHT \\sourceserver\inetpub$\wwwroot\mydir \\destinationserver\mydir /LOG:\\SomeNetworkPath\FolderCopyScript\ScriptLogs\Folder_Copy_%dt%.log