Un-fucking-believable that Kraken would actually return the error to you instead of a regular plain 400 or 500 error.
They actually returned a Python exception to you?
No, the Python errors shown are from the client-side. Kraken typically returns a JSON with the appropriate error message if something goes wrong.
@OP: It seems that something in the response message doesn't parse correctly in the JSON decoder. So take a look at the raw response message, not the Python error message, (or post it here) to see what's going on.