It seems that API is changed.
https://www.duolingo.com/vocabulary/overview
returns:(non json format)
<Response [200]>

Duolingo made a change to their Vocabulary API... I ran into the same problem with my own project. At the same time they introduced a new feature to their app, a vocabulary exercise. If I have time I will try to reverse-engineer how that exercise gets its data. I assume that they updated their vocabulary endpoint to fit that new feature, which led to a change to deprecate the known endpoint.

same problem. It seems that the vocabulary/overview api is gone.

the new feature as @Mordano has mentioned , endpoint is https://www.duolingo.com/2017-06-30/users/{id}/courses/{lang1}/{lang2}/learned-lexemes?limit=50&sortBy=LEARNED_DATE&startIndex=0.
But this new api returns much less information as compared to the former, it is a pity. The old api provides the Unit one word belongs to.

What is the state of this issue? I see a linked PR fixing this issue. Did anyone tested this?