There are two points of investigation:
1) ?Check the "Output Scan" tab to see if there is anything in the "Override host for FileScan (optionally)" settings
2) Confirm that the agent specified in the "Output Scan" tab or the "Attributes" tab has access to read the file specified in the "Output" tab.
This happens if the Agent specified in the "Attributes" tab does not have access to read the file specified to be registered (either via script or the "Output" tab).
This can also occur if the "Output Scan" tab has an agent in the "Override host..." section that does not have access to read the file specified.
Being sure that both of these have access should resolve this issue.?