The feed for “The Vintage News” provides each <item> with a relative URL and NewsBlur doesn’t use the <channel>'s <link> to complete the URL, instead using NewsBlur’s own URL to do so.
The Vintage News feed URL: https://www.thevintagenews.com/feed/
Example:
<channel>
<title>The Vintage News</title>
<atom:link href="https://www.thevintagenews.com/feed/" rel="self" type="application/rss+xml" />
<link>https://www.thevintagenews.com</link>
<description>The Vintage News site</description>
...
<item>
<title>Did Self-Help Become A Thing Due To 19th-Century Radicals?</title>
<link>/2021/05/27/did-self-help-become-a-thing-due-to-19th-century-radicals/</link>
...