Support for Drupal 7 is ending on 5 January 2025—it’s time to migrate to Drupal 10! Learn about the many benefits of Drupal 10 and find migration tools in our resource center.
Line 2609 of node.module says
$node->rss_elements = array(
array('key' => 'pubDate', 'value' => gmdate('r', $node->created)),
If you have a locale other than "en_US" on your server you might get a pubDate that is not a valid RSS PubDate. Your rss.xml is not quite valid according to http://validator.w3.org/feed/ and will not work everywhere.
https://drupal.org/node/812260 offers a couple of solutions (#0 and #4) for the views RSS module. Solution #0 may be the best fit for the node.module?