The Promotion Plugin uses the custom workspace of the project since JENKINS-8547 and there is no option to choose a different workspace, in case the promotion step runs on a different node (Property 'Restrict where this promotion process can be run' in the plugin is different to project property 'Restrict where this project can be run').
Lets say the build ran on a Windows node and the custom WORKSPACE there is something like 'C:\folder1\folder2'. If we define the promotion step to run on a UNIX node, then Jenkins tries to create 'C:\folder1\folder2' on a UNIX machine. And there is no way to prevent this.