The copyartifacts plugin looks in the archive directory of the referenced project. If this does not exist because the job did not have "Archive the artifacts" selected, the reported error does not indicate this. The error as emitted is:
Started by user anonymous
Unable to access workspace for artifact copy. Slave node offline?
Build step 'Copy artifacts from another project' marked build as failure
Only after attaching a debugger did we discover the full path that the plugin was using for the workspace and realized we needed to enable artifact archiving.