You have additional code active there add this string inside the feed and destroy the XML markup. The hook on this place is rss_tag_pre (see https://github.com/WordPress/WordPress/blob/b3b8942dfcb451eddb5559b63c1043fce5d9449e/wp-includes/feed-rss2.php#L21).
Deactivate all plugins, use a default Theme, and then, step by step, activate it again to identify the source for this problem. Alternative search about the wp-content directory for the hook rss_tag_pre.
Also, make sure there is no custom RSS feed in place in your functions.php.