After finally updating to Ubuntu 17.10, my SMB network shares couldn’t mount. The magic sudo mount -a only returned a marvelously unhelpful “mount error(95): Operation not supported” message.
Excessive Googling led me to discover that “they’ve changed the default dialect to SMB3”. Whoever “they” are, I have a small reasonable rant some feedback: This isn’t Debian Unstable, yeah? Please don’t break shit like this too often.
To fix SMB ‘dialect’ issues, you need to edit your /etc/fstab file to add vers=1.0 to the comma-delimited list of mount options. After which, sudo mount -a will restore your network shares.