issue was:
net share from Windows 10 to windows XP failed to work, out of the blue





two ways to solve this:


via gui:
control panel - add remove programs - add remove windows features 
(note that win server puts the add remove windows features in that server gui page)

enable this and the two children
SMB 1.0/CIFS File Sharing Support
 - SMB 1.0/CIFS Client
 - SMB 1.0/CIFS Server


via cmd line:

Set-SmbServerConfiguration -EnableSMB1Protocol $false


of interest to note is that WinXP suddenly stopped being able to see the share
no immediate / visible error message that indicated the root of the problem

based on my knowledge and experience, I was able to determine the SMB1.0 resolution.



ways to troubleshoot this further without guessing:
watch packets in wireshark. see where communications stops between client and server.
test against other servers to narrow down where problem was (in this case on win10 client)
(this is what I consider isolating the problem. follow the signal path)
consider past history of this setup (and recent changes)