https://superuser.com/questions/850301/mount-error5input-output-error-on-mount
When setting up a share on a Windows 2012 server, I had to use:
sudo mount.cifs //server/folder /path/to/folder/ -o user=username,password=pass,vers=3.0
Note the vers=3.0.
Based on the suggestions below, you may need to try vers=2.0. The vers parameter is the key.