I just had the same error. The cause of the problem for me was that I didn't have depends_on formatted corretly. This may not be the same problem for you, but it might be worth checking similar things. The good news is that it won't spoil your adventure with any unwanted hints in the error message.

I was using: "depends_on": ["A"]
It was fixed by switching to: "depends_on": [{"task_key":"A"}]