HTML error while connecting between Appian to Appian instances.

Question

I have webAPI in Instance A expecting a small set of json payload(4-5 field), nothing crazy.
I have Integration build on Instance B to call the WebApi of instance A. Passed the API keyfrom Instance A as needed to instance B.
WebAPI --> API Key --> Integration etc.
I am receiving weird error, looks like some authentication issue but is coming in HTML format and could not get more info from logs either. 
Saying JSON is invalid but JSON is valid.
Can someone please help me on this?
HTML error received: 
                                     document.addEventListener('DOMContentLoaded', function () {document.forms[0].submit()})                                                 Note: Since your browser does not support JavaScript,                 you must press the Continue button once to proceed.                                                                                                                                                                                                                    
WebApi from Instance A:

Integration calling above WebAPI:

Connected system called inside integration:

stefanhelzle0001 · Answer

This happens when your credentials are wrong. I think your header name is wrong. Check the documentation here: https://docs.appian.com/suite/help/24.2/Web_API_Authentication.html#authentication

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