Hi Farhan,
You are almost there! Can you post a screenshot of the communication channels using HTTP and HTTP_AAE? What does the trace log in SMICM say (only for Plain HTTP)? To get the log, execute your scenario and then go to tcode SMICM in PI, select display all and scroll to the downmost part. It should give a hint as to why you are getting a 400 error.
Hope this helps,
Mark