The DHL server (API) responds with this message if the EKP (DHL account number) or the participation numbers (Teilnahmenummern) are not correct or missing.
To resolve this, please follow this tutorial:
Shipments to Austria in Magento 2
If the problem occurs in Magento 2 and only affects shipments from Germany to Austria, the product "DHL Connect" might not be enabled in your DHL account.
To process shipments to Austria with "DHL Paket International", the following quickfix can be applied.
Open the file "vendor/dhl/lib-shipping-mx/Util/ShippingProducts.php".
Search for this code section and remove it completely:
ShippingRoutes::COUNTRY_CODE_AUSTRIA => [ self::CODE_CONNECT, ],
Afterwards, flush all Magento caches, including the static files. Otherwise the code change might not have an effect.