Rest POST request is failing due 400 bad request error, though it's working fine with Postman

I'm doing a POC with ReadyAPI to check the suitability of the tool for use as API automation tool for my company.
There is a scenario where I need to store the full JSON response from one GET API and pass it as part of a POST JSON request body.
I was trying to do the JSON transformation via the Property Transfer feature in ReadyAPI but it doesn't work.(Response is saving fine but still I'm getting 400 bad request when I'm passing the payload.)
To find the root cause of the issue, I copied the working JSON body from Insomnia and pasted it in the ReadyAPI body with all the other required other details, and that time also its failed. But the same JSON is working with Postman without any issue and I'm getting the expected response.

The JSON response that I'm passing is having more than 2000 lines and it's altogether closer to 2400 lines of request body.

Any idea to resolve this issue? Tried to open a case but ReadyAPI new support request page is not working. 

I know it's really hard to find the root cause of this issue with the limited information. I would like to see if there is anyone who faced the similar issue and fix it.
Error
<Fault xmlns="https://xxxxxxxxxxxxxxxxx/v2/cart/-1/products">
<code>40000010000000000</code>
<message>Problem with validating input.</message>
<details>Problem with reading HTTP message.</details>
<errors>
<e>API Error Code : 1 - Validation Failed</e>
</errors>
</Fault>