Feed url

When I generate the static files, I get a /feed dir for every page and post. However, when google crawls my site and asks for /feed/, it receives a 403 (forbidden) error. If I use the full address, like https://www.justthink.it/feed/index.xml all works, while https://www.justthink.it/feed/ doesn’t work
How can I solve the “google” problem? (I think other readers my be involved in the issue)

The page I need help with: [log in to see the link]