My RSS feed (and all variations of it, ie – atom, red, rss, rss2, etc..) had been giving problems for quite some time, showing up as “not a valid feed” at validator.w3.org/feed and inaccessible through my feed delivery service Dlvr.it which I use to automatically share my posts to Facebook, Twitter etc.. I tried re-uploading all the WordPress core files manually in case any of them had become corrupt. I also tried editing out white/blank space in important theme files like functions.php and wp-config.php but the problem persisted.
Eventually, a deactivation of all of my plugins fixed it but rather than turn on each one of the 50 or so plugins I use one by one to find the culprit, I tried a quick google search for something like “wordpress plugin break rss” and landed on this support thread for the WordPress HTTPS plugin which I’d been using for a while:
Sure enough, disabling that plugin sorted the issue but I thought I needed it. As it happened, I’d already converted my whole site over to HTTPS so didn’t really need the plugin’s functionality any more so I could safely leave it off. The plugin automatically parsed all non HTTPS url’s in the WordPress content which was handy and the only thing I was missing with it turned off but with a little tweak of my .htaccess file I was able to redirect all HTTP urls to HTTPS anyway.
The fix at the above thread might work for some temporarily but it’s not ideal considering plugin updates will overwrite the changes. I’ve submitted a report request to the developer and will update this post if he/she replies!