Unable to access files with File Sharing in Mac OS High Sierra
Gordan's got the issue: 
To connect to an APFS formatted volume you need to turn OFF sharing via AFP (forcing SMB connection) on the macOS devices you are trying to share. 


Open System Preferences (Screen Menu > Apple icon > System Preferences
Select Sharing icon (3rd row, Folder w/ caution sign)
Select File Sharing in Right hand list (check to turn on if not already)
Select Options ... botton on right
UNCHECK Share files and Folder using AFP
Repeat as needed for Mac devices you're trying to share


Then you should be able to connect and log in to the Mac shared on you network as before.

See Apples (very) brief doc Prepare for APFS in macOS High Sierra for other items affected by the change to APFS.