If Calibre is set to require authentication, it must use "basic" authentication, not "digest" authentication. Digest authentication is the default for unencrypted communications, but KOReader doesn't support digest authentication. See [issue #3953](https://github.com/koreader/koreader/issues/3953). **Warning**: Unencrypted basic authentication is *insecure*. Anyone who can inspect your network traffic will be able to see your password. Only use basic authentication with SSL or when it's only run on a completely-trusted network without being exposed to the Internet.