oh lol. that’s literally it? 
.
Do you get the same output using like cURL or Postman directly? Anything helpful in the response headers?
I’m fairly certain this isn’t an ESI issue, but something with routing your request to it, as normally ESI would return JSON error messages, even AWS would return HTML. Seems that error is related to cloud flair. Where are you geographically? Could be triggering some protections or something 
, hard to say.