Magento – Is it possible to override the Session Cookie Lifetime found in the sys. config.

If no section request parameter is present, the currently active section is simply the first one in the ordered list of system.xml sections.

This happens in Mage_Adminhtml_Block_System_Config_Tabs::initTabs().
(I have removed parts of the method for the sake of clarity!)

public function initTabs()
{
    $current = $this->getRequest()->getParam('section');
    $configFields = Mage::getSingleton('adminhtml/config');
    $sections = $configFields->getSections($current);
    $sections = (array)$sections;
    usort($sections, array($this, '_sort'));
    foreach ($sections as $section) {
    if ((empty($current) && $sectionAllowed)) {
        $current = $code;
        $this->getRequest()->setParam('section', $current);
    }
    // ... leave out the rest of the method ...

The _sort() callback method simply sorts the sections by <sort_order> value.
So which section is sorted first?

As it happens, thats the <paypal> section, which is configured in Mage/PaypalUk/etc/system.xml!
This section has no <sort_order> node, so it is sorted to the top.

But, you may ask yourself, why isn't the section shown?

Well, the section configuration is not only missing the <sort_order> declaration, it is also missing the <show_in_default>, <show_in_website> and <show_in_store> settings.
And the default, if these are not set explicitly to be visible, is to hide the part of the user interface (code found in Mage_Adminhtml_Block_System_Config_Form::_canShowField()).

If we where to add <show_in_default>1</show_in_default> to the <paypal> section, the default page would no longer be empty, it would show the Website Payments Pro (Payflow Edition) group.

If you want a different section to show up as the default, just give it a <sort_order> that sorts before an empty string ;)

In previous versions of Magento the <paypal> section had a sort order set, and - if no custom module messed it up - the default section would be the General section from the Mage/Core/etc/system.xml file.
I believe (but don't know) that the current behavior with the blank page is a bug.

1. Check if time (date and timezone as well) are set correctly on your server.
2. Check if Base URL of your installation is set correctly at System > Configuration > Web
3. Check if your sessions can be saved on filesystem or in DB. Open app/etc/local.xml file and look for <sessions> section, it is either set to files or db. Make sure that files can be written under var/session or in database and there is enough disk space or inodes.
4. If you use any third-party caching extension (turpentine or FPC or cache extensions that utilize varnish etc.), try disabling these extensions. To completely disable extension edit its XML config in app/etc/modules/Extension.xml and change <active> tag to false.