Potential security threat when posting to api/v5/recognitions

I'm creating a .NET 5 application pointing at a sandbox environment. I have the authentication process completed successfully. I am able to retrieve a list of users and the options list for the recognitions endpoint. When attempting to POST a recognition to api/v5/recognitions, an html page is returned stating that a potential security threat has occurred and the recognition is not created. Can you share possible causes for this error and how to get around it?

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