AWS API Gateway CORS pre-flight check fails

I'm using Zappa to deploy my application, cors is enabled and everything seems to work OK when pressing the "test" button on the options in API Gateway "OPTIONS" resource.

However, when I try to do the CORS pre-flight check, I get a 500 error with body {"message": "Internal server error"} and CloudWatch logs Execution failed due to configuration error: Unable to transform request

EDIT:

This is what the configuration looks like:

EDIT 2 I've tried enabling the CORS by both via zappa_settings.json ("cors": true) and by manually clicking enable cors in the AWS console

Does anyone have any pointers how to debug this further?