An invalid WordPress RSS Feed could be caused by a number of issues. I’m going to point you to several troubleshooting options and the one in particular that worked for me. The thing to remember is that the issue is with the RSS feed, so you want to target files/functions within plugins and themes that would likely be editing the RSS feed.
You can check the validity of your RSS feed by entering your WordPress RSS feed URL at https://validator.w3.org/feed/. Alternatively, when you’re using an RSS feed for an automated email newsletter, the newsletter provider may validate before allowing you to enter in your RSS feed. This is what happened to me, the RSS feed wasn’t accepted by MailChimp, so I had to make it valid so that it would be accepted.
Deactivate the Most Likely Plugin to be Interacting with the WordPress RSS feed:
It’s possible that one of the plugins that is installed on your WordPress site is interacting with the RSS feed. In order to detect which plugin is the culprit, go through and deactivate each plugin that you think is likely to be causing the issue and then test the RSS feed after deactivation. If the RSS feed is suddenly valid, then you know the most recent plugin that you deactivated is the problem.
This was the issue for me. I installed a plugin called RSS with Images that edited the RSS Feed for my WordPress site. I didn’t have to go through and deactivate all of my plugins because I determined the most likely problem child and then deleted that plugin from my site.
Deactivate Plugins One By One:
If the above didn’t work and the limited number of plugins that you targeted didn’t validate the RSS feed, then you need to go through the rest of your plugins one by one and deactivate the plugin and test the WordPress RSS feed until you land on the guilty party.
Deactivate Theme:
It’s possible that the theme that is active on your WordPress site is also changing the RSS feed. You can quickly identify whether this is the case by de-activating your theme and re-checking the validity of your RSS feed (by going to https://validator.w3.org/feed/). If your WordPress RSS feed is still invalid, the theme is not the culprit.
However, if the WordPress RSS feed is suddenly valid, then the issue is with theme.
Fix Theme:
In this case, you can either use a different theme or fix up the issue with the theme. If you are using a custom theme then you can edit the files directly. However, if you purchased the theme, I would recommend creating a child theme first before editing any of the files directly. You can create a child theme by checking out the following post.
Any conflicts with the RSS feed are likely located in the functions.php file. So, search within that file, OR any files included or required in the functions.php file, for any mention of RSS feeds. Then, you would need to either comment out those RSS feed related functions or fix them yourself.