the thing is the sandbox is all in one and in production environment you would have multiple resource servers that can be installed on different physical servers across the network. So you need to publish those files either by using method PublishIfNeeded after you saved the file locally and to database or manually upload everything to each server respectively.
I still get the same error. I can confirm the path using the querdef method which is described in fileRes dataschema. The following gives the path of a file which has the id/primary-key as 4216 in the file resource directory.
Please find the sample code where I am trying to cope a file and paste it with another name. It is working fine for me. Please double check the path and if it is correct you can reach out to Adobe support.
var file = new File("/usr/local/neolane/nl6/var/res/instance_name/mypic.png");
logError ("File '" + file.fullName + "' does not exists.");
logInfo("File '" + file.fullName + "' exists.");
logError ("File '" + file.fullName + "' was not copied.");
logInfo("File '" + file.fullName + "' was copied.");
var copiedfile = new File("/usr/local/neolane/nl6/var/res/instance_name/halloween.png");
logError ("File '" + copiedfile.fullName + "' does not exists.");