The update in question is KB4480970, which is part of the January 2019 monthly round of patches, and addresses various security issues including delivering protection against an additional form of speculative execution side-channel vulnerability (in other words, like Spectre and Meltdown).
However, if you install this fix on Windows 7 machines (or Windows 2008 R2 Server) which are hooked up via an SMBv2 network (version 2 of Microsoft’s network file sharing protocol), it’s apparently playing havoc with network connectivity as evidenced by this post on Reddit (spotted by Windows Latest) and other chatter online.
Affected users are saying that uninstalling KB4480970 clears any problems, but obviously that’s not ideal, given that it contains a pretty important security countermeasure in the form of the speculative execution side-channel protection.
There is allegedly a fix for the Registry to get things working again after installing the patch, but as documented on the Reddit post, that doesn’t seem to work for everyone.
So it seems that your only course of action might be to uninstall KB4480970 – or just don’t install the patch in the first place – until Microsoft gets around to addressing the issue.
According to the info available on Reddit, Microsoft is currently investigating the reports of this problem. Fingers crossed that we hear something from official channels soon, and that action is taken quickly.
As ever, these major monthly updates represent something of a dilemma, because while you want all the latest security patches on your Windows PC as soon as possible for obvious reasons, it usually pays to wait at least a day or two to see if any major problems crop up.