NewsBlur does not recognise seemingly valid RSS

I’ve an order at Customer Login - Niton999, which I should be able to subscribe to using

In case that key becomes invalid, view-source: displays:

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0" xmlns:slash="">
    <title>Order # 1000111160 Notification(s)</title>
    <description>Order # 1000111160 Notification(s)</description>
    <generator>Laminas_Feed_Writer 2 (</generator>
      <title>Order #1000111160 created at 20/04/2024</title>
      <description><![CDATA[<p>Current Status: Processing-FS Acknowledged<br/>Total: <span class="price">£50.95</span><br/></p>]]></description>

It certainly appears to be valid RSS (I know, that’s potentially a misnomer, considering how unstandardized it is in comparison to its Atom successor) yet NewsBlur doesn’t accept it:


This address does not point to an RSS feed or a website with an RSS feed.