"Content body cannot be converted to new editor format" error when updating page content

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Using PUT /wiki/rest/api/content/{id} to update a page.

API call work when I send a request from Postman but not from c# code.

As an experiment, I have just get content from a page, and then update page.

If I send request from Postman it works but not from code.

I checked requests in Fiddler and they are almost identical (at least I cannot find difference)
Any ideas how can I troubleshoot it?

And give you a bit more details of how I did the test

  • it debugger I have copied and then pasted json content to Postman ( API call works)

  • in the code, only 2 statements left to make a API call

var data = new StringContent(json, Encoding.UTF8, “application/json”);
response = await _client.PutAsync(url, data); - client is a standard HttpClient

And … for some pages, everything works OK. for more complex not - for example, it does not with pretty simple table.

Any help will be appreciated.