Since the plugin also reads the affected source code files (source code view, fingerprinting, blaming, etc.) it needs to copy these files from the agent to the controller. If these files are not part of the workspace (or checked out into a sub folder of the workspace) they are not automatically found. So you can add an additional source code directory where the plugin tries to find these files. Note, that due to security restrictions additional paths outside of the workspace need to be registered in Jenkins system configuration before they can be used here.