The security update that Apple yesterday released for the vulnerability in macOS High Sierra causes issues for users that use Apple’s macOS File Sharing feature. After the update is installed, it’s no longer possible to connect to shared folders on other Macs.
Users complain on several forums that after the update they can’t connect to other machines anymore. Apple released a page on its website earlier today that explains how users can fix the issue. Apple also promised to do better next time by doing an audit on the development process. This should make sure it won’t happen again.