With the Blog Sample Data installed. click on the My Blog RSS feed. When you look in the source in the top part of the file, there is an atom tag. The link of this tag has double slashes in the URL. Also when the site is in a sub folder, this folder name get added two times.

Example 1:
<atom:link rel="self" type="application/rss+xml" href="https://localhost//index.php?format=feed&amp;type=rss"/>
Should be:
<atom:link rel="self" type="application/rss+xml" href="https://localhost/index.php?format=feed&amp;type=rss"/>

Example 2
<atom:link rel="self" type="application/rss+xml" href="https://localhost/joomladevelopment//joomladevelopment/index.php?format=feed&amp;type=rss"/>
Should be:
<atom:link rel="self" type="application/rss+xml" href="https://localhost/joomladevelopment/index.php?format=feed&amp;type=rss"/>

The problem is probably the plugins\system\sef\src\Extension\Sef.php file. On line: 226 the feed URL get replaced. Disable this part fixes the problem. This is not the solution, but a direction to get it fixed.

In PR #45592 the double slashes in the image URL's are fixed. Discussion has been started there on this bug.