Please note: This article is aimed at DHL Business Customers.
Private customers need to contact the DHL Customer Service.
The login credentials for DHL Versenden / Shipping suddenly stopped working, although there were no prior problems.
The DHL credentials do not remain valid forever. They need to be renewed / changed regularly.
This is not caused by the Magento module, but by DHL's policy.
Open the DHL Business Customer Portal (Geschäftskundenportal) and follow the instructions to renew the credentials (password). See also this article.
It is recommended to create a System User and use that for Magento. The password for such System Users is valid for a longer time.
If you run into problems, you can create a support request here.
Private customers need to contact the DHL Customer Service instead.