Problem using OData calls to read O365 calendar for user

My MVC based web app has been working fine for many months.  Today, some (but not all) users cannot access the function where the app reads their calendars.  Stack trace attached.  Any ideas?

DataServiceClientException

        Microsoft.OData.Client.DataServiceClientException: Error processing response stream. Server failed with following message:
Object reference not set to an instance of an object. ---> Microsoft.OData.Core.ODataErrorException: An error was read from the payload. See the 'Error' property for more details.
   at Microsoft.OData.Core.Json.BufferingJsonReader.ProcessObjectValue()
   at Microsoft.OData.Core.Json.BufferingJsonReader.ReadNextAndCheckForInStreamError()
   at Microsoft.OData.Core.Json.BufferingJsonReader.ReadInternal()
   at Microsoft.OData.Core.JsonLight.ODataJsonLightEntryAndFeedDeserializer.ReadFeedContentStart()
   at Microsoft.OData.Core.JsonLight.ODataJsonLightReader.ReadFeedStart(ODataFeed feed, SelectedPropertiesNode selectedProperties)
   at Microsoft.OData.Core.JsonLight.ODataJsonLightReader.ReadAtStartImplementationSynchronously(DuplicatePropertyNamesChecker duplicatePropertyNamesChecker)
   at Microsoft.OData.Core.ODataReaderCore.ReadImplementation()
   at Microsoft.OData.Core.ODataReaderCore.InterceptException[T](Func`1 action)
   at Microsoft.OData.Client.Materialization.ODataReaderWrapper.Read()
   at Microsoft.OData.Client.Materialization.FeedAndEntryMaterializerAdapter.TryRead()
   --- End of inner exception stack trace ---
   at Microsoft.OData.Client.Materialization.FeedAndEntryMaterializerAdapter.TryRead()
   at Microsoft.OData.Client.Materialization.FeedAndEntryMaterializerAdapter.TryReadFeedOrEntry(Boolean lazy, ODataFeed& feed, MaterializerEntry& entry)
   at Microsoft.OData.Client.Materialization.FeedAndEntryMaterializerAdapter.Read()
   at Microsoft.OData.Client.Materialization.ODataEntityMaterializer.ReadImplementation()
   at Microsoft.OData.Client.MaterializeAtom.MoveNextInternal()
   at Microsoft.OData.Client.MaterializeAtom.MoveNext()
   at System.Linq.Enumerable.<CastIterator>d__b1`1.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Microsoft.Office365.OutlookServices.Extensions.PagedCollection`2..ctor(DataServiceContextWrapper context, QueryOperationResponse`1 qor)
   at Microsoft.Office365.OutlookServices.Extensions.DataServiceContextWrapper.<>c__DisplayClass38`2.<ExecuteAsync>b__36(IAsyncResult r)
   at System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endFunction, Action`1 endAction, Task`1 promise, Boolean requiresSynchronization)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Microsoft.Office365.OutlookServices.Extensions.DataServiceContextWrapper.<ExecuteAsync>d__3a`2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at TBAImporterv2.Helpers.CalendarOperations.<GetCalendarEntries>d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at TBAImporterv2.Controllers.CalendarController.<ReadCalendar>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Mvc.Async.TaskAsyncActionDescriptor.EndExecute(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass37.<BeginInvokeAsynchronousActionMethod>b__36(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass33.<BeginInvokeActionMethodWithFilters>b__32(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass21.<>c__DisplayClass2b.<BeginInvokeAction>b__1c()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult)