From a clean install of Win 8.1 Pro 64-bit I had used the Hyper-V manager to test some virtualization.  After tinkering around with a few virtual machines I put that aside for a while.  Over the following weeks I continued applying updates as Microsoft released them.  Decided to play around with virtual machines again and when launching the manager I got the error "MMC has detected an error in a snap-in and will unload it."  Options are to report the error to Microsoft and shut down MMC, or unload the snap-in and continue running.  If I report the error it goes into the problem reports but as of yet there is no solution available.  If I unload the snap-in and continue running I get this additional output:
-----------
FX:{922180d7-b74e-45f6-8c74-4b560cc100a5}
Could not load file or assembly 'Microsoft.Virtualization.Client, Version=6.3.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The module was expected to contain an assembly manifest.

Exception type:
System.BadImageFormatException

Exception stack trace:
at Microsoft.Virtualization.Client.VMBrowser.BrowserSnapIn.OnInitialize()
at Microsoft.ManagementConsole.SnapInBase.Initialized()
at Microsoft.ManagementConsole.Internal.SnapInClient.Microsoft.ManagementConsole.Internal.ISnapInClient.Initialize(ISnapInPlatform snapInPlatform)
at Microsoft.ManagementConsole.Executive.SnapInInitializationOperation.OnStart()
at Microsoft.ManagementConsole.Executive.RunningOperationsTable.EnqueueOperation(Operation operation)
at Microsoft.ManagementConsole.Advanced.FrameworkSnapInFactory.Microsoft.ManagementConsole.Advanced.ISnapInFactory.CreateSnapIn(Int32 bookkeepingId, String snapInKey, Object& snapIn)
-----------

I tried restoring one of my earlier drive images and got Hyper-V manager working again.  Unfortunately continuing to run with this image wasn't feasible as I've had too many changes in the install to apply all over again, my solution needs to be repairing my image as it exists at the time I discovered Hyper-V was broken.  The older image was useful though to use to see just what causes the break.  From that restore I stepped through Windows Updates one by one, eventually finding that the fix for KB2898871, "Security Update for Microsoft .NET Framework 4.5.1 for Windows 8.1 and Windows Server 2012 R2 for x64-based Systems" was what seemed to trigger it.  Uninstalling this update on my current image does not resolve the issue.  Somehow something is hanging around that is the root cause of the problem.  An additional odd thing with this is that this KB update is marked as February 2014, yet going back through my disk images I found that the problem had existed as early as December 2013, possibly the month before.  If .NET updates are cumulative then that would make sense as an earlier update that is now included in KB2898871 could have been applied before, but I don't know if the updates are designed that way.  I have yet to find any information online directly relating to this problem and those that are somewhat similar haven't produced any working fix.  Looking for any thoughts on this while I hope that the problem report uploaded to Microsoft is being worked on.

Hi!

I've never encountered this problem, but I'm really interested to see if you get a response or find the root-cause!

In the mean time, have you tried to remove the Hyper-V role, reboot and add it back again?

Best regards
Andreas Molin

Andreas, no progress on finding root cause yet.  I haven't had time to play around more with my good image and dissecting what changes between working/not working states.  I had tried removing Hyper-V via the add/remove Windows components (or whatever it's titled under Win8, think it changed?) and unfortunately no change there.  Not entirely surprising if my suspicion is correct that it is something in the .net updates that get installed, as they exist outside of the install of the Hyper-V components.  Thanks for tossing in the idea though.

Ok, now I'm stumped.  I restored the first image I captured of my 8.1 install (11/14/2013, looks like with all available updates at that time) and started applying updates.  After this I could still launch Hyper-V without error.  Now, this image was before I had created any virtual machines and at this point I still haven't added any to my test image, but Hyper-V does launch.

So my suspicion is now moving to something about maybe the information of the VMs I had previously created in Hyper-V aren't playing well with the later .net update.  Haven't gone digging for this yet but with this new finding does anyone have any areas they think I should examine?  The first place I'm going to look is the registry and see what I can find for differences there.

I have at least created a new admin account and tested launching Hyper-V there, same error.  This confirms it is not something tied to a user ID but instead is something in a common area.

We're seeing this as well on multiple W2K machines.

You can still manage your machines by adding the snap-in manually in MMC.

Still annoying as h*ll... :-)