Magento system.xml and 404 error when trying to access the configuration panel

I'm trying to implement some configuration settings for my custom module. I've managed to add a tab and a section in the left navigation bar. But when I want to open a section I get a 404 error page without any further information.

So far, I've tried anything to get it working.. reading blogs, examples etc. but I can't find the error. Maybe someone of you can explain me what I'm doing wrong.

My adminhtml.xml:

<?xml version="1.0" ?>
<config>
    <resources>
        <admin>
            <children>
                <system>
                    <children>
                        <config>
                            <children>
                                <tempest_section translate="title" module="Tempest">
                                    <title>Tempest</title>
                                </tempest_section>
                            </children>
                        </config>
                    </children>
                </system>
            </children>
        </admin>
    </resources>
</config>

My config.xml:

<?xml version="1.0"?>

<config>
    <modules>
        <Polyvision_Tempest>
            <version>0.1.0</version>
        </Polyvision_Tempest>
    </modules>


    <global>
        <helpers>
            <Tempest>
                <class>Polyvision_Tempest_Helper</class>
            </Tempest>  
        </helpers>
    </global>        

    <admin>
        <routers>
            <adminhtml>
                <args>
                    <modules>
                        <tempest before="Mage_Adminhtml">Polyvision_Tempest_Adminhtml</tempest>
                    </modules>
                </args>
            </adminhtml>
        </routers>
    </admin>


    <adminhtml>
        <menu>
            <menu1 translate="title" module="Tempest">
                <title>polyvision</title>
                <sort_order>60</sort_order>
                <children>
                    <menuitem1 module="Tempest">
                        <title>Tempest - Export</title>
                        <action>adminhtml/tempest_main</action>
                    </menuitem1>
                </children>                
            </menu1>
        </menu>
    </adminhtml>    

    <default>
        <tempest>
            <settings>
                <export_directory>/tmp/</export_directory>
            </settings>
        </tempest>
    </default>
</config>

My system.xml:

<?xml version="1.0" ?>
<config>
    <tabs>
        <polyvision module="Tempest" translate="label">
            <label>polyvision</label>
            <sort_order>100</sort_order>
        </polyvision>
    </tabs>
    <sections>
        <tempest_section module="Tempest" translate="label">
            <label>Tempest-Einstellungen</label>
            <sort_order>200</sort_order>
            <show_in_default>1</show_in_default>
            <show_in_website>1</show_in_website>
            <show_in_store>1</show_in_store>
            <tab>polyvision</tab>
            <groups>
                <settings translate="label">
                    <label>Settings</label>
                    <comment></comment>
                    <sort_order>10</sort_order>
                    <show_in_default>1</show_in_default>
                    <show_in_website>1</show_in_website>
                    <show_in_store>1</show_in_store>
                    <fields>
                        <export_directory translate="label tooltip comment">
                            <label>My Custom Field</label>
                            <comment>Some comment about my field</comment>
                            <tooltip>Field ToolTip</tooltip>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>1</show_in_store>
                        <frontend_input>text</frontend_input>
                        <source_model>adminhtml/system_config_text</source_model>
                    </export_directory>
                </fields>
            </settings>
        </groups>
    </tempest_section>
</sections>

Well, my module itself works without hassles. Only the admin settings are not working :/