Security Monthly Quality Rollup RKB4480970 is known to break network shares when it is installed on a Windows 7 machine that is hosting the share. Uninstalling this update fixes the issue but this is not ideal as you don't benefit from other aspects of
this update not related to SMB network shares. Microsoft have issued a fix for this update fault. It can be installed manually from the Windows Update Catalogue. It is KB4487345. You can download it here: https://www.catalog.update.microsoft.com/Search.aspx?q=KB4487345.
When I first had the issue with KB4480970 I uninstalled it but didn't feel comfortable not being fully up to date. Found out about the fix, re-installed KB4480970, restarted the PC, then installed KB4487345 and restarted again. All is good once
again. All my Windows 10 and Linux PCs can see the share on my Windows 7 "server".
Additional Info for Remote Desktop Users. I use my Windows 7 PC as a headless file and print server with no screen or keyboard attached. After installing KB4480970
on my Windows 7 PC I could not see the share but thankfully I could still Remote Desktop on to it from a Windows PC (but oddly not from a Linux PC) to be able to install
KB4487345 but the remote desktop client did not log in automatically and I had to manually select a user and then type in my password.