400 Errors followed by HTML Payload! - On valid request

API Endpoint(s) and/or Zoom API Event(s)

This API is being used to register webinar attendees. Most of the time it works perfectly but every so often today it returns a 400 error which is often followed by massive payloads which appear to be Zoom web pages!

This is a log of part of the returned payload after a failure:

Content-Type: application/json
Content-Length: 158
Authorization: Bearer XXXXX (where XXXXX is an exact match to the OAUTH token received immediately prior to this call)

senddoc(): entered
{ json payload }
recvresp(): entered
HTTP/1.1 400 Bad Request

/// More cookie stuff then this payload arrives!

chunk size = 3430
get_chunk_size returned 3430
calling comm_blockread

<!doctype html>
<html xmlns:fb="http://ogp.me/ns/fb#" lang="en-US">
<head prefix="og: http://ogp.me/ns# fb: http://ogp.me/ns/fb# zoomvideocall: http://ogp.me/ns/fb/zoomvideocall#">
<title>Error - Zoom</title>
<meta http-equiv="X-UA-Compatible" content="IE=edge,Chrome=1">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="referrer" content="origin-when-cross-origin">
<meta name="viewport" content="width=device-width, initial-scale=1, minimum-scale=1.0">

I have no idea where to even begin to look for the source of this problem.

Nobody else seen this behaviour? No one from Zoom with any thoughts?