You may see this error because:
- An object in the report isn’t enabled for reporting anymore.
- A lookup relationship used by objects in the report has been deleted or modified.
- An object in the report has been deleted.
- You don’t have “View” permissions for an object in the report.
Source: http://help.salesforce.com/help/doc/en/faq_reports_error_obsolete.htm
Cheers!!!