Network connection to Macbook failed - with solution

I upgraded my iMac and Macbook Retina to High Sierra. All went well except that the iMac could not longer connect to the Macbook over the home network using Finder ( "Connection failed"). I tried numerous things until I realised that the High Sierra installation had disabled SMB file sharing. When I re-enabled SMB file sharing (on the Macbook) the connection from the iMac worked again: Systems Pref/ Sharing / File Sharing (ticked) / Options (SMB ticked)

This must have something to do with preparing for the new Apple file format.