This problem only affects DHL Shipping for Magento 2, not for Magento 1.
In Magento 2, no DHL shipping labels can be created. This error message appears:
Shipping labels could not be created: Unauthorized
The token which is needed for authentication is not valid anymore.
This is not related to the user and password from the DHL Business Customer Portal. Changing those will not resolve the problem.
Update DHL Shipping for Magento 2 to version 0.11.1 (or higher). It is available on Github.
If an update is not possible, you can manually store the new token in your system. Please create a support ticket to get the token and detailed instructions.