Ah it is easy to change the default master password of Odoo (formerly OpenERP) to something complex and then forget it.
What? You don’t know what is default master password. Oh it is just admin. But once you have changed it to a complex password it is stored in the conf file of your Odoo/OpenERP installation.
In my case I found it in /<OpenERP Root Folder>/server/install/openerp-server.conf
Change it here or copy it and don’t forget it next time.