Symptoms
Retrieved update sets are empty, meaning they do not contain any associated sys_update_xml records.
Release
All releases.
Cause
This will happen if the sys_update_set_source record for the source instance is configured to authenticate using a user that does not have the necessary roles.
Resolution
Per the documentation, update set retrieval must authenticate using a user that has the 'admin' role. Either add the 'admin' role to the specified user, or authenticate using one that has this role already.