Problem Description

Windows Sandbox can fail with "Error Code 0x800705b4" to start because of several reasons. In my case it failed after running the following command over an elevated cmd window "bcdedit /set hypervisorschedulertype core".

I reviewed today my administrative events on my PC to be sure that everything is running fine and saw the Warning event mentioned below and decided to enable the core scheduler by running "bcdedit /set hypervisorschedulertype core" command as suggested inside the event.

Log Name: System
Source: Microsoft-Windows-Hyper-V-Hypervisor
Date: 3/17/2020 10:56:02 PM
Event ID: 157
Task Category: None
Level: Warning
Keywords: (70368744177664)
User: SYSTEM
Computer: XXXXXXX
Description:
The hypervisor did not enable mitigations for CVE-2018-3646 for virtual machines because HyperThreading is enabled and the hypervisor core scheduler is not enabled. To enable mitigations for CVE-2018-3646 for virtual machines, enable the core scheduler by running "bcdedit /set hypervisorschedulertype core" from an elevated command prompt and reboot.

 
Once I enabled the core scheduler and restarted my PC Windows Sandbox did not start anymore and failed with the following error message.

Windows Sandbox failed to start.
Error 0x800705b4. This operations returned because the timeout expired.
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Solution

The fastest and easiest way to fix this problem is to revert Windows Sandbox to it’s default settings. Basically disable and enable Windows Sandbox Feature over "Programs and Features" in "All Control Panel Items".
 
Good luck !