One of Microsoft's more anticipated features coming to Windows 10 with the May 2019 (1903) update was the Windows Sandbox. Unfortunately, Microsoft admitted the recent update has left the new security feature unable to launch for many users. If that wasn't enough, Windows Defender Application Guard is also down.
Windows Sandbox was announced last year as a feature for Windows 10 Pro and Enterprise users, and would allow them to run a virtualized, sandboxed environment. This virtualization is designed to run less like a virtual machine, and more like a separate application. There's no need to install any third party VM software, as is the case with setting up a traditional virtual machine.
Windows Defender Application Guard is an enterprise-specific feature, allowing untrusted sites to be opened in a Windows Hyper-V container. This is designed to protect enterprise machines and data while employees use the network and internet.
However, Microsoft has acknowledged that both features are failing to launch with the latest KB4497936 update. In support documents, Microsoft explains a potential workaround by creating new registry keys using admin credentials, then restarting the host. Outside of that, Microsoft is working on a more permanent resolution, estimated to be available in late June.
The update also provided some protections against the newest speculative execution side-channel vulnerabilities, known collectively as MDS. It's also important to note that the KB4497936 update was only delivered to the preview rings of the Windows Insider program. That said, Microsoft hasn't exactly hit the mark with previous updates. Hopefully Microsoft can iron out the kinks, or this will be another in a long string of problematic updates.