RSS feed reports page not found

I’ve recently transferred my site from WP.com to WP.org. I use the Mystique theme with Jetpack, Akismet and Triberr plugins. Triberr is not picking up my blog posts. When I type https://gretavanderrol.net/feed into the URL, I get a 404 message. Same on Validator. Yet if I use the RSS widget on the blog, I get all my posts. I use Firefox, but I tested the rss feed URL in Safari as well.

Triberr picked up the feed just fine from WP.com.

I’ve read through quite a few forum posts looking for clues, but nothing has worked.