As we discussed yesterday, Microsoft's latest Windows 8 patch is well-worth downloading; it brings some long overdue enhancements that will make using the OS all the more enjoyable. However, as worthwhile as the update is, it appears as though Microsoft was a bit slack in testing it on the server side.
In effect, installing 8.1 Update could break WSUS' (Windows Service Update Service) ability to scan for future updates - a major headache waiting to happen for IT folk. Microsoft states that this issue applies to those who use HTTPS and SSL, and do not have TLS 1.2 enabled. The bug affects both WSUS 3.0 SP2 and 3.2.
If you're dealing with a server that's affected by this bug, Microsoft has a not-so-ideal workaround: Enable TLS 1.2, and disable HTTPS. Clearly, this workaround is not going to be adopted by too many, so we can only hope that Microsoft will issue a patch to fix the bug sooner than later. For what it's worth, the company does seem to be taking it seriously: "Microsoft plans to issue an update as soon as possible that will correct the issue and restore the proper behavior for Windows 8.1 Update KB 2919355 scanning against all supported WSUS configurations."
Given the severity of the bug, Microsoft's pulled the update from WSUS, although individual users could still update through Windows Update. For obvious reasons, that's not a route Microsoft recommends you take at this point in time.
For desktop users, you have nothing to worry about: Go and enjoy the 8.1 Update!