Notes/Domino 6 and 7 Forum : Problem populating authors fields on docs with no readers fields

We have recently installed Notes 6 on a test server, clients are still running R5. I have encountered a problem when testing a scheduled agent.

The agent clears and repopulates authors fields on a set of documents. Initially when the agent runs it successfully clears the authors fields on all documents and saves the documents. The agent then crashes with the error "You are not authorised to perform that operation" when it has repopulated the authors fields and attempts to save the first document. If you try to run the agent a second time it crashes with the same error but this time when attempting to access the first document to clear the fields. The document is still visible in the view but cannot be opened. There is a separate authors field on the document which includes the server name. This field is not amended by the agent.

If you add a readers field to all the documents and include a role which is assigned to the server and users in the ACL, then the agent runs without any problems. Is it necessary to include a readers field as well as the authors fields when using Notes 6?

Has anyone encountered this problem before and can anyone explain it?