Exception ‘Mage_Core_Exception’ with message ‘Invalid callback:

<config>
    <modules>
        <yourcompany_yourmodule>
            <version>1.0.0</version>
        </yourcompany_yourmodule>
    </modules>

    <global>
        <models>
            <yourmodule>
                <class>Yourcompany_Yourmodule_Model_Observer</class>
            </yourmodule>
        </models>
    </global>

    <crontab>
        <jobs>
            <yourcompany_mymodule>
                <schedule>
                    <cron_expr>*/15 * * * *</cron_expr>
                </schedule>
                <run>
                    <model>Yourmodule/observer::updateAll</model>
                </run>
            </yourcompany_mymodule>
        </jobs>
    </crontab>
</config>

 <crontab>
        <jobs>
            <yourcompany_mymodule>
                <schedule>
                    <cron_expr>*/15 * * * *</cron_expr>
                </schedule>
                <run>
                     <model>yourcompany_yourmodule/observer::updateAll</model>
                </run>
            </yourcompany_mymodule>
        </jobs>
    </crontab>