Problem
This error message appears in the log:
Could not access SOAP webservice
No DHL Express products are shown in the checkout.
Solution
First, update the DHL Express extension and also the DHL Express SDK to the latest version. Use Composer to execute the update.
If the problem persists, check the log file again. Newer extension/SDK versions will give more meaningful error messages in the log.
Lastly, please go through this checklist.