This thread looks too old but I came across to similar issue.

I am trying to copy millions of files from 1 server to another over network.

When I use the robocopy code without /mt, it looks working fine. But when I add /mt, /mt:2 etc. it stuck on same screen as above. Ram usage increasing. I have waited 20 minutes but nothing happened. It just copied the folders but not the files inside. This happens in win server 2016.

Anyone may suggest something ?