I took the example Atom feed from https://sample-feeds.rowanmanning.com/examples/64968fb0e3685ef07a3e2a27b8d64c01/, added subtitle with   and it can be found at https://sixthform.info/feed.xml
I can add this feed successfully to v1.24.3 but it fails in 1.26.3 with this logged error (#force_feed didn't work either)

https://sixthform.info/feed.xml is invalid XML, likely due to invalid characters. XML error: Undeclared entity error at line 7, column 37 [https://sixthform.info/feed.xml]

This is the content of feed.xml:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">

 <title>Example Feed</title>
 <link href="http://example.org/"/>
 <subtitle>Welcome to the M208&nbsp;Group Theory 1&nbsp;discussion forum.</subtitle>
 <updated>2003-12-13T18:30:02Z</updated>
 <author>
   <name>John Doe</name>
 </author>
 <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

 <entry>
   <title>Atom-Powered Robots Run Amok</title>
   <link href="http://example.org/2003/12/13/atom03"/>
   <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
   <updated>2003-12-13T18:30:02Z</updated>
   <summary>Some text.</summary>
 </entry>

</feed>

Originally posted by @stevem6 in #7514