Hi,
I'm recommending www.service.sap.com/notes (and even better the x-search) for looking for knowledge base articles (these aren't indexed in google for example) - it 's a gated service, you'll need an S-user.
Why not take a look at Note 1685693 - Error:
“Return code 6” while saving the report to the BW server.
..To get you started?
Best of luck
Regards,
H