Unable to add https://greenparty.org.uk/feed/. The W3C Feed Validation Service says "This is a valid RSS feed", with zero warnings: 
http://validator.w3.org/feed/check.cgi?url=https%3A//greenparty.org.uk/feed/

STEPS TO REPRODUCE
1. Click Feed / Add Feed
2. In Feed URL box enter https://greenparty.org.uk/feed/ and click OK
3. Popup error box says "Feed not found from https://greenparty.org.uk/feed/."

I tried with the --addfeed option. I first renamed ~/.local/share/akregator to ~/.local/share/akregator.bak. This was the result:

$ akregator --addfeed https://greenparty.org.uk/feed/
"https://dot.kde.org/rss.xml"
"https://www.linux.com/feed/"
"https://planet.kde.org/global/atom.xml"
"https://store.kde.org/content.rdf"
"https://planet.kde-espana.org/atom.xml"
"http://planet.ubuntu.com/rss20.xml"
"http://feeds.feedburner.com/ubuntu-fridge"
"http://www.kubuntu.org/news/feed"
"unexpected character" on line 1
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = akregator path = /usr/bin pid = 678451
KCrash: Arguments: /usr/bin/akregator --addfeed https://greenparty.org.uk/feed/
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi

[1]+  Stopped                 akregator --addfeed https://greenparty.org.uk/feed/

===

Perhaps the problem is that line 1 of the feed has both the xml version and the rss version. The feed starts:

<?xml version="1.0" encoding="UTF-8"?><rss version="2.0"
    xmlns:content="http://purl.org/rss/1.0/modules/content/"
    xmlns:wfw="http://wellformedweb.org/CommentAPI/"
    xmlns:dc="http://purl.org/dc/elements/1.1/"
    xmlns:atom="http://www.w3.org/2005/Atom"
    xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
    xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
    >

<channel>
    <title>Green Party</title>
    <atom:link href="https://greenparty.org.uk/feed/" rel="self" type="application/rss+xml" />
    <link>https://greenparty.org.uk/</link>
    <description>Real hope. Real change.</description>
    <lastBuildDate>Wed, 19 Jun 2024 07:54:06 +0000</lastBuildDate>
    <language>en-GB</language>
    <sy:updatePeriod>
    hourly  </sy:updatePeriod>
    <sy:updateFrequency>
    1   </sy:updateFrequency>
    <generator>https://wordpress.org/?v=6.5.4</generator>
    <item>