Thank you everyone for the guidance and help. After it became apparent that it was unlikely this was a samba issue, I spent some time today searching for possible causes in Windows.
I ran across the article below, followed the solution, and both desktop computers were immediately able to access the samba share.
Article with Solution: https://superuser....
If smb.conf file modifications have no effect, then most probably the issue is on those two windows 10 machines side. It seems right even because of the fact that the one machine with windows 10 is able to connect to your Ubuntu share. It most probably means that nor firewall and nor samba services reject connections. But because of there are many different ...
It's getting crowded in here ....
There are 4 ways a Win10 machine can access an Ubuntu Samba server:
 By ip address = the "cleanest" way since it doesn't rely on any intermediate protocol. You would need to set a static ip address on the server or use your router to do that and you would need to access the server explicitly \\192.168.0.100
Follow these instructions. For maximum usability make sure the SAMBA account username (separate from server username) on the server matches that of the Windows account you're using to access the share.
One more smb.conf configuration I tried and it seems working in my windows 10 VM you could try:
server max protocol = SMB3
encrypt passwords = yes
dns proxy = no
strict locking = no
oplocks = yes
deadtime = 15
max log ...
I could not try it now on Windows, but it works from ubuntu at least. This is a config I've taken from here and I think it worth of try:
Backup your current /etc/samba/smb.conf file by copying it: sudo cp /etc/samba/smb.conf /etc/samba/bk.smb.conf
Edit your /etc/samba/smb.conf:
workgroup = WORKGROUP
server string = %h server (Samba, Ubuntu)...
To check the content of odd folder and have ability to remove it, use terminal.
In terminal window go to your folder using a cd command: cd /home/$USER/UbuntuShare
Check this odd file's owner and permissions by ls -ailh command
If you really want to remove that file, you could use sudo rm -fr /pathto/UbuntuShare/core
But it is better to examine what is ...