Get Modelview Metadata : “Failed to query the data.”

Much like the question raised here I get "Failed to query the data" when calling GetModelviewMetadataAsync from the .net API. The stack trace is

Autodesk.Forge.Client.ApiException
  HResult=0x80131500
  Message=Error calling GetModelviewMetadata: {"diagnostic":"Failed to query the data."}
  Source=Autodesk.Forge
  StackTrace:
   at Autodesk.Forge.DerivativesApi.<GetModelviewMetadataAsyncWithHttpInfo>d__38.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Autodesk.Forge.DerivativesApi.<GetModelviewMetadataAsync>d__37.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at forgeViewer.Controllers.ModelDerivativeController.<GetMetadataInternal>d__7.MoveNext() in D:\Users\xxxx\Documents\Repos\xxxxx\xxxxxxx\Controllers\ModelDerivativeController.cs:line 169
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at forgeViewer.Controllers.ModelDerivativeController.<Webhook>d__5.MoveNext() in D:\Users\xxxx\Documents\Repos\xxxxx\xxxxxxxxxx\Controllers\ModelDerivativeController.cs:line 136
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__139_1(Object state)
   at System.Threading.QueueUserWorkItemCallback.<>c.<.cctor>b__6_0(QueueUserWorkItemCallback quwi)
   at System.Threading.ExecutionContext.RunForThreadPoolUnsafe[TState](ExecutionContext executionContext, Action`1 callback, TState& state)
   at System.Threading.QueueUserWorkItemCallback.Execute()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()

  This exception was originally thrown at this call stack:
    [External Code]
    forgeViewer.Controllers.ModelDerivativeController.GetMetadataInternal(string) in ModelDerivativeController.cs
    [External Code]
    forgeViewer.Controllers.ModelDerivativeController.Webhook(Newtonsoft.Json.Linq.JObject) in ModelDerivativeController.cs
    [External Code]

The complication is that we are processing multiple files and we are calling GetModelviewMetadataAsync after a translate webhook completes so there should be no concurrency issues.

How to diagnose this issue?

Thanks