This error message occurs:
Class Dhl\Shipping\Model\Attribute\Backend\TariffNumber does not exist
The previous DHL Shipping module (up to version 0.11.1) has not been removed completely. There are still entries in the DB which need to be removed.
Make a backup of the database and then remove the old entries from the legacy DHL Shipping module, see these instructions in the readme.
To prevent this problem altogether, please follow the instructions for the current module, chapter "Migration from previous DHL Versenden extension". Observe the order of the steps and the installation/removal of the DHL modules!