Customers are experiencing a severe bug in KB4480970 where computers are unable to access network shares. Reports have been rolling in steadily since the roll-up for Windows 7 was released.
Microsoft has now updated the roll-up history page to reflect the bug, issue some reasoning, and apply some guidance for affected customers.
Local users who are part of the local “Administrators“ group may not be able to remotely access shares on Windows Server 2008 R2 and Windows 7 machines after installing the January 8th, 2019 security updates. This does not affect domain accounts in the local “Administrators” group.
To work around this issue use either a local account that is not part of the local “Administrators” group or any domain user (including domain administrators).
We recommend this workaround until a fix is available in a future release.