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As for this specific error message:

Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed. Disconnect all previous connections to the server or shared resource and try again.

The problem here is that when a user on Windows first makes contact with a server it automatically passes that users' local login username. If you have a public share it remembers that it took that username successfully. When you try to connect to a private share and don't pass the exact same username and password, it thinks you are trying to access with another name.

Some workarounds:

1) Map the Windows user to the local Linux user - this is useful for Win10 users who log into their systems with a Microsoft account.

Create a file at /etc/samba/smbusers

Map the local Linux user to the Windows user by adding the conversion to the file. For example:

luci = luci@example.com john = "John Smith"

Then add a reference to the map in /etc/samba/smb.conf in the [global] section:

username map = /etc/samba/smbusers

Finally restart smbd: sudo service smbd restart

2) Make another connection to the Linux server

Windows can address a Linux server in two (three in Win10) different ways. By hostname, by ip address, or by mDNS name (Win10 only):

\ubuntu\Protected \192.168.0.100\Protected \ubuntu.local\Protected

Even though all of these point to the same machine, Windows sees them a three separate hosts.

3) Map the "network drive"

When mapping a drive, select the "connect using different credentials" option.