I recently came across the problem where I was posting CSV data to a HTTP Endpoint and I was getting a 422 "Unprocessable Entity" http status code you can look up the HTTP status code definition with the following link.
what was happening is that my payload was not being sent, testing the same payload with Postman seemed to work fine to the same URL with the same headers so it was a behaviour in Boomi that was effecting it.
the issue was that I had a dynamic resource path and part of which I specified to be a variable
on the connector I was populating this as a parameter which i assumed to be correct
however this is not correct, when you create the resource path is clearly states "is replacement variable?" as a check box and the keyword here is variable not parameter.
to set this as a variable you need to insert a Set Properties shape and set a dynamic document property to the same name as you have specified in the the resource path, in this instance ID.
insert this after your map shape, in my case the message shape, and before the connector shape and remove the parameter and it should look like this.
How I found out how to solve this problem was by reading another community thread and discovered in the comments a discussion on the above issue and how to solve it and I thought it might be best to break this out so its easier to find for people experiencing the same issue. You can read the original thread here.