If you were on GitHub I would probably just file a pull request, but since you are not, the following code in https://svn.jenkins-ci.org/trunk/hudson/plugins/copy-to-slave/src/main/java/com/michelin/cio/hudson/plugins/copytoslave/CopyToMasterNotifier.java looks wrong:

public boolean perform(AbstractBuild build, Launcher launcher, BuildListener listener) throws InterruptedException, IOException {
...
    FilePath projectWorkspaceOnSlave = build.getProject().getWorkspace();

This is likely wrong since the project can have many workspaces, and if there are concurrent builds this one may be from a different build. build.getWorkspace() is more correct, and simpler too.