The error happens on the start of kibana. According to the developers it is benign. I am getting the same error, so it does not seem related to the transform problem.
If you want to further investigate using developer tools, it would be nice to have it open before you send the request. After you executed a request you can check in the network tab what has been send. A comparison between the one that works and the one that does not would be helpful.
I am sorry that this is all a bit vague, a support case - if you eligible for support - might work better.