This solved my issue and I was able to train the model. Thank you so much! You are great, Chris!
Maybe a little hint in the documentation would have been helpful, since I just started experimenting with Rasa. For this reason it was not clear to me and I mistakenly focussed only on the explicit error message. But even then in my opinion for a beginner it is still hart to decipher that the problem is rooted within the flow_retrieval feature.