Same problem here Daniel. Are you in germany? Are you also using the Germanized plugin? Cause in their last update they integrated the DHL into the Germanized plugin, and maybe that is why the DHL plugin stop working. Also make sure to change the password in the website from DHL because you need to change it every 3 months.
I hope someone here can help us because this tool is crucial for the shop I manage. There is many orders all the time.
Many thanks, m.