the problem with the URL is the usual i.e. a non-standard port. there’s plenty of threads here which discussed this so i’m going to refer you to search.
After futher investigation and looking at a specific post in your link, I found the issue: the SSL certificate.
I have no idea how my synology handle it (or even how apache uses it), but I now make the calls to an http URL instead of https. I also edited tt-rss to use http and no https.
Due to an automatic http to https synology redirection, I can’t use tt-rss web version anymore. But the feed is properly working, I can even use tt-rss android app.
I don’t fully undertand why the feed works and not the web, but it works.
Thanks for your help and for making such a great tool I’ve been using for a many years now.