I add youtube channel link on the web app, for example https://www.youtube.com/@3blue1brown
It show me an error and HTTP 403 Forbidden message.
I dont know what happened at background, but it seems work again.
YouTube’s RSS feeds are very finicky. When I was a user of a different reader app, I would often find I couldn’t subscribe to YouTube channels’ RSS feeds; it wasn’t because of the app, but because the feeds themselves were hidden/rate-limited by Google. If I wait an hour or so and try to subscribe again, it usually worked. Maybe that’s what you experienced?