Withings API subscription always returns 293
I'm trying to set up a subscription on the Withings API. (all other calls to the API are authenticated and produce valid returns). The subscription always returns "293 - the callback URL is either absent or incorrect. I've searched existing questions and everything seems to check out; tried http and https, my URL exists and testing in Postman shows it returns 200 - Ok and responds in 263 ms. What am I missing here?