Error on API-created posts

I’ve got a problem with all posts I’ve created as drafts throught the API. I’ve used the MobileDoc format to create them, and included an embedded card.

On each of the posts, when trying to edit them through the UI (I’m assuming on auto-save), I get an error like (only the URL changes between posts):

Ember AJAX Request PUT returned a 406 Payload (text/html) [Omitted Lengthy HTML]

If I try and publish any of these, I get the message “Unknown error”.

The posts show up fine in a query against the API, so the data is there and seems to be broadly correct. Any ideas how I can fix this? I’m using the hosted platform.

Here’s an example of the mobiledoc info on a post that shows the problem that I get back from the API:

{"version":"0.3.1","atoms":[],"cards":[["embed",{"url":"","html":"<iframe width=\"200\" height=\"113\" src=\"\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture\" allowfullscreen></iframe>","type":"video","metadata":{"title":"","author_name":"Matthew Bellringer","author_url":"","height":113,"width":200,"version":"1.0","provider_name":"YouTube","provider_url":"","thumbnail_height":360,"thumbnail_width":480,"thumbnail_url":""}}]],"markups":[],"sections":[[10,0],[1,"p",[[0,[],0,"Recorded at: 13:00, 03 Nov 2020"]]],[1,"p",[[0,[],0,"Delightful Dissent is a webinar series about joyfully challenging assumptions. It's an hour of conceptual creative destruction. My guest shares a common saying or idea. We take it apart, then put it back together again in a more helpful way.      In this introductory episode to Delightful Dissent, we explore how exceptional people challenge assumptions, why that’s important, and how we can all do more of it. Unlike every other episdoe, Matthew is going to be the one being interviewed!  “The exception that proves the rule” is commonly used to explain away things that don’t fit. It’s almost as if the existence of a contradiction makes the rule more reliable. That makes no sense to Matthew.       Matthew Bellringer is a consultant, facilitator, speaker and author. He works with people who cultivate transformation. People who do magic for individuals, groups and organisations. They are misunderstood precisely because of the qualities that enable them to make this kind of change. Matthew helps these people unpick these misunderstandings, reclaiming and then working from their unique capability to motivate profound change.      Remeny, our guest interviewer, is the co-founder of Brilliant and Human. She works with businesses to turn their clients into advocates.  We will get to know her more in Episode 3, where the tables will be turned and Matthew will be speaking to her.  If you're watching this event live please share your perspective or ask questions in the comments. We'll see them as you post them and there will be time at the end of the hour to answer any questions that come up.    Matthew's website -  The Delightful Dissent blog -"]]],[1,"p",[]]],"ghostVersion":"4.0"}

Looks like this error is relates to the iframe element. It happens when I use HTML instead of Mobiledoc, and I even get a similar error when I try and enter an iframe as an HTML element in the Ghost Admin editor.