Microsoft: December Windows Server updates break Hyper-V VM creation

Microsoft says Windows Server updates released during December’s Patch Tuesday will trigger errors when creating new virtual machines on some Hyper-V hosts. 

The known issue affects only Windows Server/AzStack HCI hosts in SDN-enabled environments that are managed using System Center Virtual Machine Manager (SCVMM).

“You might receive an error on workflows involving creating a new Network Adapter (also called a Network Interface Card or NIC) joined to a VM network or a new Virtual Machine (VM) with a Network Adapter joined to a VM network,” Microsoft explains.

“Existing VMs with existing Network Adapters should not have issues connecting after installing this update, only new Network Adapters created after installation of this update are affected.”

On affected systems, Windows admins will see one of the following errors:

• New VM creation fails: Creating a new VM with Network adapter fails.
• SLB Load Balancer or SDN RAS Gateway fails with an ‘EthernetConnection’ error
• Attaching vNIC fails: Attaching a Network adapter to an existing VM.  
• Warning message during live migration: Live migration of Non-Highly Available VMs completes with an ‘EthernetConnection’ warning message.

Microsoft added that only Windows Server 2019 and Windows Server 2022 should be impacted after installing this month’s Patch Tuesday updates (KB5021237 and KB5021249).

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Workaround available

Microsoft has also provided admins of affected SDN-based SCVMM deployments with a workaround for this newly acknowledged issue.

It requires running the following set of commands on all SCVMM-managed Hyper-V hosts from an elevated PowerShell window (immediately after installing the KB5021237 and KB5021249 updates):

$lang = (Get-WinSystemLocale).Name

C:Windowssystem32wbemmofcomp.exe C:Windowssystem32wbemen-USVfpExt.mfl

C:Windowssystem32wbemmofcomp.exe C:Windowssystem32wbemVfpExt.mof

Admins can also do this after installing the updates using a dedicated script for bulk deployment of the workaround on the SCVMM Management Server if the number of affected hosts is too large and running the commands manually would be too time-consuming.

Microsoft also provides a post-install script that can be used with patching tools when deploying the KB5021237 and KB5021249 updates.

You can find the two scripts for large-scale deployments and further details on applying the workaround in this support article.

Redmond said that it’s now working on a fix for the known issue and will provide an update with a future release.