Big thanks to jak who brought our broken feed to our attention in a comment he left here. We have managed to rectify the problem and fixed our feed so it can be read in feed readers.
The problem was that somehow something in the theme is adding blank lines to the top of our feed which caused the xml to be not well-formed and thus invalid. It seems that the problem was with blank lines before <?php and after ?> in wp-config.php and the theme’s functions.php.
We managed to fix the broken feed with these resources.
Blank line before XML declaration (WordPress)
WordPress Support topic
I believe the feed has been broken for a long time and no one told us about it, except jak. We love you jak!