HI Krishna,
HTTP VBO does not show message details if the HTTP Response code is 500. i observed it during the one of my project, i think you still have two ways to get error details,
1. Use a third party API consumption tool like Postman(Free) or SOAPUI(Paid), you can send same API Request through any of this tool and you would be able to read exact error message.
2. Try to update your HTTP VBO, generally we update the tool but do not update the inbuilt objects, i think the HTTP VBO which comes with BP6.4..3 or later can display error details when the HTTP response code is 500.
I hope this would help.
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Ishan Mahajan
India
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