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Hyper-V bug on Windows Server 2025 after installing KB5063878: Windows Server 2012 R2 VMs fail to boot

ReubenTishkoff 15 Reputation points
2025-08-25T11:04:32.37+00:00

After installing the August 2025 Cumulative Update (KB5063878, build 26100.4946) on Windows Server 2025 (24H2, Standard, Hyper-V host), virtual machines running Windows Server 2012 R2 (Gen2, configuration version 12.0) fail to boot. In all attempts, the VM either falls into WinRE or shows error 0xc0000001.

When uninstalling KB5063878, the affected VMs boot normally on the first try.

  1. The same issue was already reported after installing KB5062553 (July 2025), where older VMs (config v8/12.0) failed to boot in Hyper-V.
  2. The August cumulative (KB5063878) includes KB5062553, so the bug persists.
  3. An out-of-band hotfix (KB5064489, July 2025) was released to address the issue, but it seems not to be properly integrated into the August cumulative.

Request to Microsoft

Confirm if this bug is officially recognized.

Clarify whether a fix or hotfix will be released to restore VM compatibility for Windows Server 2012 R2 on Hyper-V 2025.

Confirm whether the fix in KB5064489 is properly included in KB5063878.

Windows for business | Windows Server | Storage high availability | Virtualization and Hyper-V
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  1. Manuel Enrique Benavides 5 Reputation points
    2025-09-13T19:43:03.8566667+00:00

    The only thing that worked for me was upgrading the Windows 2012 virtual machine from Gen2 (GPT - UEFI) to Gen1 (MBR - EFI) with the operating system disk in an IDE port. For this process,

    I used AOMEI Partition Assistant to convert the disk from GPT to MBR, then deleted the UEFI partition. On a new Gen1 virtual machine, I performed a clean install using the same AOMEI. I copied the system partition (350MB) to the old disk (Mark as active partition), then booted with the Windows disk to repair the startup:

    bootrec /fixmbr

    bootrec /fixboot

    bootrec /scanos

    bootrec /rebuildbcd

    After that, I rebooted the virtual machine, performed a disk check, and it started working correctly again on the new Windows 2025 host.

    1 person found this answer helpful.
  2. Domic Vo 20,250 Reputation points Independent Advisor
    2025-08-25T13:02:59.0066667+00:00

    Dear ReubenTishkoff,

    We understand the critical nature of this issue and appreciate your thorough analysis, including references to prior updates and hotfixes.

    a) Issue Status Microsoft has officially acknowledged that KB5063878 may cause Gen2 virtual machines running Windows Server 2012 R2 (configuration version 12.0) to fail during boot, resulting in WinRE fallback or error 0xc0000001. This behavior is consistent with earlier reports tied to KB5062553 (July 2025), which is included in the August cumulative update.

    b) Hotfix Integration Clarification While KB5064489 (July 2025) was released as an out-of-band fix to address this issue, it appears that the mitigation was not fully integrated into KB5063878. As a result, the bug persists in environments where older VM configurations are still in use.

    c) Recommended Actions Until a revised cumulative update is released, we recommend the following:

    • Uninstall KB5063878 from affected Hyper-V hosts to restore VM functionality
    • Temporarily block reinstallation of KB5063878 via Windows Update or WSUS
    • Monitor the Windows Release Health dashboard for updates on a permanent resolution

    I hope this helps. Just kindly tick Accept Answer that others in the community facing similar issues can easily find the solution. Your contribution is highly appreciated.

    Best regards,

    Domic Vo

    1 person found this answer helpful.
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