Microsoft fixes new bugs that break Hyper-V virtual machines in Windows Server

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Earlier this week, Microsoft released the December 2022 Patch Tuesday updates for various supported versions of Windows. Now, the company has warned that the latest Windows Server updates could prevent IT administrators from creating new virtual machines (VMs) on some Hyper-V hosts.

After installing KB5021249 on Software Defined Networking (SDN) managed Hyper-V hosts configured with System Center Virtual Machine Manager (VMM), you may get an error on workflows that involve creating a new network adapter (also called network interface or NIC) joined to a VM network, or a new virtual machine (VM) with a network adapter joined to a VM network, Microsoft explained on the Windows Health Dashboard.

In particular, IT administrators may encounter Ethernet connection errors when creating a new virtual machine or a network adapter on an existing virtual machine. Also, these errors may appear if the SLB Load Balancer or SDN RAS Gateway services fail. Administrators can also see Ethernet connection warning messages during real-time migrations of highly available virtual machines.

The issue only affects new NICs created after installing the latest Patch Tuesday updates (KB5021237 and KB5021249) on Windows Server 2019 and Windows Server 2022. However, it does not cause connection issues for existing VMs with NICs .

Microsoft provides a workaround to fix Hyper-V issues in Windows Server

Thankfully, Microsoft says its engineers are currently working on a fix that should arrive in the near future. As a workaround, Microsoft suggests users run the following commands in PowerShell with administrator privileges:

$lang = (Get-WinSystemLocale).NameC:\Windows\system32\wbem\mofcomp.exe C:\Windows\system32\wbem\$lang\VfpExt.mflC:\Windows\system32\wbem\mofcomp.exe C:\ Windows\system32\wbem\VfpExt.mof

Microsoft has provided a dedicated script to help organizations with large scale deployments on the SCVMM management server. Additionally, there is also a post-install script that can be used with patching tools. IT administrators can download both scripts from this support page.

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