This is a new one for me, I have not seen this in the log before as far as I am aware
[2017.06.19] 11:37:35 System.ArgumentException: Source array was not long enough. Check srcIndex and length, and the array's lower bounds.
[2017.06.19] at System.Array.Copy(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length, Boolean reliable)
[2017.06.19] at System.Collections.Generic.Queue`1.SetCapacity(Int32 capacity)
[2017.06.19] at System.Collections.Generic.Queue`1.Enqueue(T item)
[2017.06.19] at MailStore.Indexing.PendingIndexDeletions.AddItem(String emailAddress, IndexDeletionItem item)
[2017.06.19] at MailStore.Indexing.UserIndexCoordinator.#eik.#Thk(KeyValuePair`2 #bcd)
[2017.06.19] at System.System_ExtensionMethods7BCA73B06BAB478aA3AC6AC60979BA25.Each[T](IEnumerable`1 enumerable, Action`1 action)
[2017.06.19] at MailStore.Indexing.UserIndexCoordinator.#9kb(String #bab)
[2017.06.19] at MailStore.MailStoreEventNotifier.CalendarItemUpdated(String emailAddress)
[2017.06.19] at MailService.Protocols.ActiveSync.Helpers.SyncCalendar.SaveItems()
[2017.06.19] at MailService.Protocols.ActiveSync.Helpers.SyncBase.#Loe(Dictionary`2& #EZi, Dictionary`2& #FZi, Boolean& #GZi)
[2017.06.19] at MailService.Protocols.ActiveSync.Helpers.SyncBase.PerformSync(Int32 allowedChanges, Boolean& requiresImmediateResponse, Boolean& saveLastSyncRequest)
[2017.06.19] at MailService.Protocols.ActiveSync.Commands.Sync.Execute()
[2017.06.19] at MailService.Protocols.ActiveSync.VersionSpecific.VersionHandlerBase.ProcessCommand()
[2017.06.19] at MailService.Protocols.ActiveSync.ActiveSyncProcessor.ProcessCommand(EasCommandRequest request)