Getting `Unsupported request - method type: post` when trying to create an ad account with the Facebook API

I'm trying to create an ad account in my Facebook Business with this API endpoint:

https://developers.facebook.com/docs/marketing-api/reference/ad-account/

I'm using these parameters for my API call:

{   "name": "Graph API Test 123",   "currency": "EUR",   "timezone_id": "42",   "end_advertiser": "{my_business_id}",   "media_agency": "{my_fb_app_id}",   "partner": "NONE",   "funding_id": "{my_funding_id}" } 

According to the docs, a POST endpoint is available and valid.

When removing the funding ID from my request it works. I've checked if the payment method behind the funding ID is active and valid and it is. It's assigned to other ad accounts as well and I'm spending money with it.

Does anyone know why this happens? I'm pretty sure it used to work in the past.