Update: after hours of troubleshooting, we managed to identify the problem. In case it would help anyone else, the source was the following:
We had a Feedburner WordPress plugin we’ve always been using. It is called “FeedBurner FeedSmith”. We deactivated that plugin.
Then we deleted our browser cache and also Cloudflare’s cache of our site, then tried the RSS links again. I am delighted to report they seem to be finally working now and redirecting to our new RSS feed provider.
A tool you may find useful is redirectdetective.com .