VMM Migration Error 20413 (Hyper-V-VMMS 20770)

TL;WR: The VMM errors are usually pretty generic, you almost always have to look at host logs. This specific instance it was an SMB error which was relatively surprising. If you couldn’t find any issues in the usual log spots, try looking at the SMBClient logs on the host.

I was trying to migrate a VM from one of our less-used staging hosts when I started getting an exception at the Live Migration step.

----------------------------------------------------
--------------- Bucketing Parameters ---------------
----------------------------------------------------
EventType=VMM20
P1(appName)=vmmservice.exe
P2(appVersion)=4.0.2413.0
P3(assemblyName)=ImgLibEngine.dll
P4(assemblyVer)=4.0.2413.0
P5(methodName)=Microsoft.VirtualManager.Engine.ImageLibrary.HyperVHAVM.AddDiskResourceToVMFromFilePath
P6(exceptionType)=System.NullReferenceException
P7(callstackHash)=a724

SCVMM Version=4.0.2413.0

...

Base Exception Assembly name=ImgLibEngine.dll
Base Exception Method Name=Microsoft.VirtualManager.Engine.ImageLibrary.HyperVHAVM.AddDiskResourceToVMFromFilePath
Exception Message=Object reference not set to an instance of an object.

...

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.VirtualManager.Engine.ImageLibrary.HyperVHAVM.AddDiskResourceToVMFromFilePath(String path, IVmmDbConnection dbConnection)
   at Microsoft.VirtualManager.Engine.VmOperations.DeployVmBase.MigrateVM(IVmmDbConnection dbConnection)
   at Microsoft.VirtualManager.Engine.VmOperations.DeployHost2Host.RunSubtask(IVmmDbConnection dbConnection)
   at Microsoft.VirtualManager.Engine.TaskRepository.SubtaskBase.Run(IVmmDbConnection dbConnection)
   at Microsoft.VirtualManager.DB.SqlContext.Connect(Action`1 action)
   at Microsoft.VirtualManager.Engine.TaskRepository.Task`1.SubtaskRun(Object state)

On the source host I saw a number of SMBClient errors with ID 30905:

The client cannot connect to the server due to a multichannel constraint registry setting.

Server name: \<TARGETHOST>

Guidance:
The client attempted to use SMB Multichannel, but an administrator has configured multichannel support to prevent multichannel on the client. You can configure SMB Multichannel on the client using the Windows PowerShell cmdlets: New-SmbMultichannelConstraint and Remove-SmbMultichannelConstraint.

Short answer was that the source server had some constraints (Get-SmbMultichannelConstraint) and I was in a position where I could just temporarily disable multichannel (Set-SmbClientConfiguration -EnableMultiChannel $false). Realistically the right answer would have been to validate the configuration and get it working correctly, but this host was up for decommissioning so we let it slide.