Try setting the log level of “REST Consume” to “TRACE”. This will log whatever is being sent and received when you call the remote REST service. You should now be able to see what the problem is in your logs. If you’re still not sure, share the request and the response here and we try to help some more.
I would Suggest to use custom Template and pass the Json in the body.
By Using ExportToMapping Activity in microflow you can convert Mendix objects into a String.
This helps you to see the Json that is generated and validate it manually if it properly structured or not.