Microsoft is investigating a known issue preventing the Windows Sandbox and Windows Defender Application Guard apps from opening on some Windows 10 devices. The issue affects Pro and Enterprise editions of Windows 10, version 1903, 1909, and 2004. Microsoft says that it’s currently working on a resolution to address this known issue, with a fix to be provided as part of a future release. The two apps will throw the following errors after failing to open on affected systems: Error_VSMB_SAVED_STATE_FILE_NOT_FOUND (0xC0370400)
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-working-on-a-fix-for-windows-10-sandbox-failing-to-open/