Earlier this week, a bug in macOS High Sierra was discovered that made it possible to use the root user without a password. That could be dangerous in combination with malware.
Thankfully, Apple is resolving this bug within 24 hours with a security update. Only this has created a new problem that affects a select number of users. If you use file sharing within your own network, this may stop working. You can easily solve this problem with a Terminal command.
Solution when File Sharing is no longer working
• Open Terminal (Finder ▸ Programs ▸ Utilities)
• Copy the following command (command⌘ + C)
• Paste the command into an empty terminal window (command⌘ + V)
• Press ‘enter’
Now enter your user password (from the account that is logged in), it is normal that no characters appear. Press ‘enter’ to execute the command. From now on file sharing works again and you can use it again as usual.