"Couldn't find feed" for valid RSS feed: softies.net

[omgitsraven]

The webcomic https://softies.net/ has an RSS feed at https://softies.net/category/comics/feed/ which is apparently valid RSS: https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fsofties.net%2Fcategory%2Fcomics%2Ffeed%2F

but when I try to add that URL to Capy Reader, the popup just says "couldn't find feed".

Is there any kind of log where I could read more detail about what caused it to reject the content?

[omgitsraven]

I just discovered that if I swap HTTPS for HTTP, CapyReader can subscribe to it fine. Which is odd, because nothing else seems to complain about the server's HTTPS.