If your administration page is throwing an error or showing you the blank page there could be two reasons behind it. Sometimes it's related to shipping method detection and sometimes it's related to payment method detection. The reason behind those problems are usually errors in payment or shipping methods (failure in instantiating them properly) .

Magento 1.7.0.0:

This is related to a Magento bug with missing DHL configuration variables (see more http://www.magentocommerce.com/bug-tracking/issue?issue=13702)

To solve this please go to System > Configuration > Shipping methods > DHL and save the configuration. The error is related to fact that config values are not existing in your database and requiring a list of all shipping methods (for default shipping method choice) will fail cause of missing config values and error is displayed or report is given.

If this does not solve the issue then you can try and remove (if it is there)

<dhlint>
<content_type>D</content_type>
</dhlint>

from app/code/local/Idev/OneStepCheckout/etc/config.xml

Older versions:

To solve the problem:

• please open up the app/code/local/Idev/OneStepCheckout/etc/system.xml
• find the block and comment it out or remove completely

<default_shipping_method translate="label">...</default_shipping_method>

• if this does not solve your problem then find the block and comment it out or remove completely

<default_payment_method translate="label">...</default_payment_method>

• either one of those blocks or both together are causing the issue
• cache cleaning is a must after etc/*.xml file editing if cache is enabled

Note: * commenting out in xml means adding blocks between following tags

This removes the Default Payment Method and Default shipping method options from OneStepCheckout administration panel. If it's troubling you then you might try to resolve the errors that certain payment or shipping extensions are creating. Usually they are failing in module definitions (like no $_code for payment methods in module settings)