Why can't I mount shares by SMB since High Sierra
Solution 1:
I was encountering the same error "mount_smbfs: server rejected the connection: Authentication error" when attempting to connect via command line using the commands:
mkdir /Volumes/test
mount_smbfs //user@server/test /Volumes/test
However, I was able to elimate the error by going, on the server Mac, to System Preferences - Sharing - File Sharing - Options. Then under Windows File Sharing, click to enable the less-secure password option for the account you wish to use for sharing (optionally, create a sharing-only user for this purpose), and enter that account's password. I was then able to mount the share using the above commands.