After the upgrade to the 8.1.x branch I was unable to connect to my "NAS" a Asus router with a USB hard-disk using samba.
I fixed it for me, and posting this as reference for people who might run into the same problem.
I tried all options suggested like "fixing" the SMB protocol version but none of them worked. I was able to find the NAS but when I wanted to connect it kept prompting for the password.
I disabled the password security on my NAS to test if that was the issue, and it was able to connect.
As neither Kodi nor the logs gave me any useful debug information I tried connecting to the NAS "manually" by using the SSH command-line.
LibreELEC-MeMoBox:~ # smbclient -L <host> -U <user>
Enter WORKGROUP\users's password:
Server does not support EXTENDED_SECURITY but 'client use spnego = yes' and 'client ntlmv2 auth = yes' is set
session setup failed: NT_STATUS_ACCESS_DENIED
This gave me something to work with, I googled and apparently my NAS isn't having the extended security samba is expecting so I need to disable it.
By adding "client use spnego = no" and "client ntlmv2 auth = no" to the config file. I added them to /storage/.config/smb.conf and it worked for the manual command-line connecting.
However Kodi was still unable to connect. As I found out it does not use the "system" instance of samba, but its own with its own config file.
However you are able to customize it, so I did the following and that fixed my connection problems:
TL; DR;
Created file: /storage/.kodi/.smb/user.conf
Added the following lines:
#Samba client options
client use spnego = no
client ntlmv2 auth = noWorks a treat 
Now, as a warning, you are disabling some "extra security" with these lines, so use with care.