I'm having this error when I try to create a new database and installing module sales.
When Odoo generates the currency rates on install it generates from the first day of year for example: 2015-01-01.
But in the sale/sale_demo.xml there are some sales that uses for the date_order this:
datetime.today - relativedelta(months=1).
As the date_order is prior do currency rate date it raises the error associated to the currency.
Anyone has experienced similar issues?