https in your FreshRSS feed URL.
I recently started using RSS again, after the Reddit API pricing fiasco. My app of choice for RSS reading on both my iPhone and Mac is Reeder 5. It’s great, has pleasant animations, and everything. It’s the best overall RSS reader for the Apple platform.
I have two accounts for all my RSS feeds: A self-hosted FreshRSS instance, and one using the built-in iCloud syncing in Reeder.
For some reason, when I tried to add my FreshRSS account to Reeder, it would show me the error „An error occurred while trying to log in.“ After some googling, I came across this GitHub issue, but it wasn’t that relevant.
After getting no help from Google and Github, I tried something completely random: I noticed that the automatically-generated API URL from Reeder started with http instead of https. And that tuned out to be the problem.
After I changed http in the URL to https, the feed got added, and now works properly!