Success! The problem was that in Windows 10 you can use two different passwords to log on to your account if you use your Microsoft (Hotmail, etc.) account password. You can use the MS account password and also create PIN to log in, giving you two passwords. If you are using the MS account or PIN to log in locally, to log into the share from a remote machine, you must use the MS account password and local user name.
This is a very handy tool, thanks to Stefano for posting it:
cd /mnt
mkdir w10test
mount -t cifs -o username=user,password=yourpassword //windowsmachineIP/share ./w10test
DON'T use the admin web site to troubleshoot connection problems for backup. It is takes too much time. Use the above commands from PuTTY.