https://wccftech.com/fixed-windows-10-1903s-windows-sandbox-black-screen-issues/

Microsoft delivered a cumulative update to Windows 10 May 2019 Update earlier today. The update brings a long list of general fixes and quality improvements to the latest version of the operating system. It also finally resolves the Windows Sandbox issue that was first spotted in May right after the release of version 1903.

The Windows Sandbox would fail to start with an error code “0x80070002” or ERROR_FILE_NOT_FOUND on devices in which the operating system language is changed during the update process when installing Windows 10 May 2019 Update, version 1903.