After upgrading Unity to 6.4 (6.4.5f1) from 6.3 (6.3.10f1), we can't drag&drop anything in hierarchy, even trying to drag from project to hierarchy it doesn't work and an error appears.
It seems that in 6.4 Unity changed how entityID works, now instead of a normal int (that Spine uses on SpineEditorUtilities lines 227 and 519) now uses int64 instead.
Tried on a new proyect (6.4.5f1) with only the latest Spine runtimes installed, and it fails just trying to move an object in hierarchy.

Here is the error:

ArgumentException: Object of type 'System.UInt64' cannot be converted to type 'System.Int32'.
System.RuntimeType.CheckValue (System.Object value, System.Reflection.Binder binder, System.Globalization.CultureInfo culture, System.Reflection.BindingFlags invokeAttr) (at <f713e1c3c5f545ce9b1df47230f4f853>:0)
System.Reflection.RuntimeMethodInfo.ConvertValues (System.Reflection.Binder binder, System.Object[] args, System.Reflection.ParameterInfo[] pinfo, System.Globalization.CultureInfo culture, System.Reflection.BindingFlags invokeAttr) (at <f713e1c3c5f545ce9b1df47230f4f853>:0)
System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <f713e1c3c5f545ce9b1df47230f4f853>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <f713e1c3c5f545ce9b1df47230f4f853>:0)
System.Delegate.DynamicInvokeImpl (System.Object[] args) (at <f713e1c3c5f545ce9b1df47230f4f853>:0)
System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) (at <f713e1c3c5f545ce9b1df47230f4f853>:0)
System.Delegate.DynamicInvoke (System.Object[] args) (at <f713e1c3c5f545ce9b1df47230f4f853>:0)
UnityEditor.DragAndDrop.Drop (System.Int32 dropDstId, System.Object[] args) (at <5cd1e59afb2c420d8621a96062f56dea>:0)
UnityEditor.DragAndDrop.DropOnHierarchyWindow (UnityEngine.EntityId dropTargetEntityId, UnityEditor.HierarchyDropFlags dropMode, UnityEngine.Transform parentForDraggedObjects, System.Boolean perform) (at <5cd1e59afb2c420d8621a96062f56dea>:0)
Unity.Hierarchy.Editor.HierarchyGameObjectHandler.DoHandleDrop (Unity.Hierarchy.HierarchyViewDragAndDropHandlingData& data, System.Boolean perform) (at <c8cd0ca2a22d4a078cdc59400644a704>:0)
Unity.Hierarchy.Editor.HierarchyGameObjectHandler.Unity.Hierarchy.IHierarchyEditorNodeTypeHandler.CanDrop (Unity.Hierarchy.HierarchyViewDragAndDropHandlingData& data) (at <c8cd0ca2a22d4a078cdc59400644a704>:0)
Unity.Hierarchy.HierarchyViewDragHandler.HandleNodeHandlersDrop (Unity.Hierarchy.HierarchyViewDragHandler+HierarchyViewDragAndDropTargets dragAndDropTargets, UnityEngine.UIElements.DragAndDropData dragAndDropData, Unity.Hierarchy.HierarchyNode& parentNode, System.Boolean perform) (at <6239b889a44b41e2b7d28eb827fcfb60>:0)
Unity.Hierarchy.HierarchyViewDragHandler.GetVisualMode (UnityEngine.UIElements.HandleDragAndDropArgs& args) (at <6239b889a44b41e2b7d28eb827fcfb60>:0)
Unity.Hierarchy.HierarchyViewDragHandler.DragAndDropUpdate (UnityEngine.UIElements.HandleDragAndDropArgs args) (at <6239b889a44b41e2b7d28eb827fcfb60>:0)
UnityEngine.UIElements.HierarchyV2.CollectionView.RaiseHandleDragAndDrop (UnityEngine.Vector2 pointerPosition, UnityEngine.UIElements.DragAndDropArgs dragAndDropArgs) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.HierarchyV2.CollectionViewDragger.GetVisualMode (UnityEngine.Vector3 pointerPosition, UnityEngine.EventModifiers modifiers, UnityEngine.UIElements.HierarchyV2.CollectionViewDragger+DragPosition& dragPosition) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.HierarchyV2.CollectionViewDragger.UpdateDrag (UnityEngine.Vector3 pointerPosition, UnityEngine.EventModifiers modifiers) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.DragEventsProcessor.OnDragUpdate (UnityEngine.UIElements.DragUpdatedEvent evt) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.EventCallbackFunctor1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0) UnityEngine.UIElements.EventCallbackRegistry+DynamicCallbackList.Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0) UnityEngine.UIElements.EventDispatchUtilities.HandleEventAcrossPropagationPath (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target, System.Boolean isCapturingTarget) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0) UnityEngine.UIElements.EventDispatchUtilities.DispatchToCapturingElementOrElementUnderPointer (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, System.Int32 pointerId, UnityEngine.Vector2 position) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0) UnityEngine.UIElements.MouseEventBase1[T].Dispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (UnityEngine.EntityId entityId, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (UnityEngine.EntityId entityId, System.IntPtr nativeEventPtr) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (UnityEngine.EntityId i, System.IntPtr ptr) (at <9e4c8e7022d34c0b8fe6be9c7b3c22df>:0)
UnityEngine.GUIUtility.ProcessEvent (UnityEngine.EntityId entityId, System.IntPtr nativeEventPtr, System.Boolean& result) (at <acda01d4940e4954ba798bbe5ce71a57>:0)!<