The login credentials for DHL Versenden / Shipping suddenly stopped working, although there were no prior problems.
"login failed / Anmeldung fehlgeschlagen"
The DHL credentials need to be renewed / changed after some time. They do not remain valid forever.
This is not caused by the Magento module, but by the DHL gateway (API).
Open the DHL Business Customer Portal (Geschäftskundenportal) and follow the instructions to renew the credentials (password).
It is recommended to create a separate System User for the API. The password is valid for a longer time for such System Users. See also this article.