RSS-канал называется index.xml
вместо rss.xml
.
> list.files(pattern = "[.]xml", recursive = TRUE)
[1] "public/categories/index.xml" "public/categories/r/index.xml"
[3] "public/index.xml" "public/post/index.xml"
[5] "public/sitemap.xml" "public/tags/index.xml"
[7] "public/tags/plot/index.xml" "public/tags/r-markdown/index.xml"
[9] "public/tags/regression/index.xml"