HTTP 403 on Index Retrieval

When the index is retrieved on the first request, it fails with:
```
HTTP Error 403: Forbidden
Fetching RFC index failed. Connected to the internet? Behind proxy?
```
I am connected though, and can open `https://www.rfc-editor.org/in-notes/rfc-index.xml` in the browser.

I think one should be able to reproduce this by deleting the cache file so the the index has to be downloaded again.