Hope the report is not reverted to induvidual or you dont have permission to view that. First try to query against the database to confirm that.
Did you modify the reports? i.e did u downloaded the rdl file (from the report Account Overview / its sub report) from any other environment and uploded here?
These inbuilt CRM report Parent and child relationship will be mapped with GUID (only for CRM inbuilt report, since during the migration process the GUID are newly created.
Try to open the report rdl (parent and child report) in Bussiness intelligence studio and fix the parent and child relationship. Once uploaded the problem will be solved.