DeepL API doesn't function any longer - API key not recognized

We are running Studio 2019 SR2 15.2.8.3007. Suddenly, the DeepL plug-in doesn't accept the API key. Removed everything with DeepL context (credential bin etc.) under AppData/user/roaming (also local) etc. and re-installed newly downloaded plug-in. Still, the API key is not accepted ... and yes, the subcription is active.

What is the solution?