I have found your case curious, so i tried to reproduce it. And yes, I finally got the same error.
One of the possible reasons is that you do not need to define payload at request level.
I have created Book model with Studio, it is json model.
My business process looks like that:
Finally it looked like that:
The Create Book task defined this way:
The script of Create book task (_bookObject is created with Create context object task)):
Request that performing book creation, defined as (it can be PUT or POST, it does not matter):
And, finally, the connector definition:
So I can suppose, you had some mess in the create request payload definition. Maybe, some typo, as I had.
I used error method with log to differentiate between standart debug messages in console from one hand and because of the debug messages are filtered from the script, on the other hand.
Hope it will help somebody!