Connection reset error when trying to post Markdown and preview in posts

[flexseth]

Originally reported as ​https://core.trac.wordpress.org/ticket/65360, posted here by @dd32

---

​https://make.wordpress.org/core/2026/05/21/media-editor-modal-call-for-testing/#respond

Add Markdown, try to preview
it times out.

Seen this on other .org properties

Error:

The connection to the server has been interrupted. Please reconnect.

[justlevine]

The ​https://make.wordpress.org/core/wp-admin/admin-ajax.php?action=o2_read&method=preview endpoint returns a 400 with the following payload:

{
    "success": false,
    "data": {
        "error": "cannot_edit_posts",
        "errorText": "Sorry, you are not allowed to edit posts on this site."
    }
}

[softglaze]

Still reproducing on 21 August 2026, on ​https://make.wordpress.org/core/2026/08/20/call-for-wordpress-7-1-x-release-managers/

Three details that may be worth adding.

The request fails fast rather than timing out. The admin-ajax.php?action=o2_read&method=preview call @justlevine identified returns a 400 in around 500ms. The neighbouring admin-ajax.php?action=o2_read&method=poll calls on the same page all return 200 in a similar time, so the connection itself is working.

The 400 payload is unchanged since May. It still returns {"success":false,"data":{"error":"cannot_edit_posts","errorText":"Sorry, you are not allowed to edit posts on this site."}}

The error message shown to the user does not match what happened. "The connection to the server has been interrupted. Please reconnect." appears in response to that 400, which makes a server-side rejection look like a network fault. I spent several days testing across different browsers, incognito windows, networks and IP addresses before finding this ticket.

Posting the comment itself works normally in the same session. Only the preview request fails.

Environment: Chrome 151.0.7922.169, Windows 11 Pro N (25H2), logged in, tested in a normal window and in incognito.