In Automated Test Framework when you add a step to 'Send REST Request - Inbound' you will receive a '400 Bad Request' error if the system property 'glide.server.url' is set to a value different from the instance you are currently testing on.
The most likely cause of this issue is when you have cloned a production instance over a sub-production instance and the clone has updated the 'glide.server.url' value to the URL of the Production instance rather than the SubProduction instance.
You then would not notice this until you create your ATF 'Send REST Request - Inbound' and execute it. You would then see the 400 Bad Request error as well as a reference to the Production URL in the ATF step result logs.
The fix is just to update the property to the correct URL of the instance.