I've spent all day so far trying to resolve this. File sharing just doesn't work any more!
I am now fairly certain that installing Security Update 2017-001 for macOS High Sierra to fix the “root” vulnerability problem (see: https://www.imore.com/macos-has-root-bug), is the cause of my file sharing problem! C'mon Apple, where's your quality control!
Does anyone know how I can fix this?