As a temporary workaround, could you move that copy Command into a batch file and verify if that works?
Many of us follow the habit of running commands via version-controlled shell scripts that are used as materials. This decouples the commands from the orchestration tool, provides version control, enables us to take advantage of a script file rather than just a single command, use a variety of scripting languages, keep the gocd configuration focused on orchestrating script execution, reduces change in the gocd configuration, etc.
So if you use a script file as a material and get it to work, please do consider continuing to use the script file
As for the reason the custom command failed, is that a custom command or a task in a job? I have a vague recollection that Chad fixed some stuff related to Windows paths earlier this year but he could confirm or the change log could show it.
— Sriram