This error occurs if the wrong DHL configuration section has been used / enabled in Magento.
In the admin panel at "Configuration -> Sales -> Shipping methods" you will find several sections called "DHL". It is important to configure the DHL Versenden/Shipping extension.
You must only use the section "DHL Versenden" (see image below). In the other DHL sections, the setting "Enabled" must be set to "No".
If the configuration was done correctly, you will not see a shipping method "DHL" in the checkout, which is normal. For more details about this, please read this article.