Unfortunately, when you have a failed webhook call Marketo doesn't show you the payload that was sent. Sort of counterintuitive!
But debugging from the sort of HTML-ified version of the payload isn't so great anyway.
You should set up a mock endpoint (ngrok, httpbin) etc. and send the payload there. It's way better to see everything as it truly is on the wire. I've had to debug webhooks where it came down to UTF8-encoded CRs vs CRLFs. You have to see the raw payload to catch that.
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