this happened to me today:
UndoManager History.Capacity is set to 30.

"Index was out of range. Must be non-negative and less than the size of the collection.\r\nParameter name: index"
   at System.Collections.CollectionBase.System.Collections.IList.get_Item(Int32 index)
   at MindFusion.Diagramming.TableNode.RowCollection.get_Item(Int32 index)
   at MindFusion.Diagramming.TableNode.x82dfc06fddeef289(Int32 xa806b754814b9ae0, Int32 xf0476e906b9d9af4, Boolean x86f283d396b4d73c, RectangleF& xc6aa8d689752bed6)
   at MindFusion.Diagramming.TableNode.GetSpannedCellRect(Int32 row, Int32 col)
   at MindFusion.Diagramming.TableNode.x82dfc06fddeef289(Cell xe6de5e5fa2d44af5)
   at MindFusion.Diagramming.TableNode.Cell.x070dff02744c711e()
   at MindFusion.Diagramming.TableNode.x541a631ae8b4bd87()
   at MindFusion.Diagramming.TableNode.x69e7e7f794373897(xae79ba7265cba94d xa17c9656698d5114)
   at MindFusion.Diagramming.Commands.RedimTableCmd.Undo()
   at MindFusion.Diagramming.Commands.CompositeCmd.Undo()
   at MindFusion.Diagramming.CommandHistory.xc411ac8d6adbe651()
   at MindFusion.Diagramming.UndoManager.Undo()
 

It might be related tothis topic.

Yes, I use them for defining subsections in TableNodes.

Also this happened to me today after this update (tableNode.Rows.RemoveAt(pos) in one tableNode:

   at System.Collections.CollectionBase.System.Collections.IList.get_Item(Int32 index)
   at MindFusion.Diagramming.TableNode.RowCollection.get_Item(Int32 index)
   at MindFusion.Diagramming.TableNode.x92dd3bdbfcc3d333(IGraphics x41347a961b838962, Pen x6bbc908355c90c11, Brush x03bd6f528805a66d, RectangleF xa6236fc5cd405c4e, RenderOptions xdfde339da46db651)
   at MindFusion.Diagramming.TableNode.Draw(IGraphics graphics, RenderOptions options)
   at MindFusion.Diagramming.Diagram.x83e1efc9aa69ada9(IGraphics x41347a961b838962, DiagramItem xccb63ca5f63dc470, Boolean x9c41bb4d9b3b314f)
   at MindFusion.Diagramming.Diagram.x679ae26de3befa07(IGraphics x41347a961b838962, RectangleF xd0772cd9ac472b96, Boolean x979174172ae18db3)
   at MindFusion.Diagramming.Diagram.Draw(IGraphics graphics, RenderOptions options, RectangleF clipRect, Boolean noModifiedItems)
   at MindFusion.Diagramming.WinForms.DiagramView.x45babdc9db40342f(IGraphics x41347a961b838962, RectangleF xd1cff1e8f8666dbe, Boolean x8de543c74cdd4f6e)
   at MindFusion.Diagramming.WinForms.DiagramView.OnPaint(PaintEventArgs pe)
   at System.Windows.Forms.Control.PaintWithErrorHandling(PaintEventArgs e, Int16 layer, Boolean disposeEventArgs)
   at System.Windows.Forms.Control.WmPaint(Message& m) 

I have also found, that tableNode.RowCount value can be different with tableNode.Rows.Count.

When using tableNode.Rows.RemoveAt(pos); and then reading from tableNode[1, pos], the previous vaue is still there - some updates are not working between RowCount, Rows and tableNode[]... it's then quite complicated to update items in tableNodes, for example if each row represents one object by it's displayname, then renaming one object should be reflected in tableNode - remove object on previous position and inserte it into new position based on it's new displayname. That's currently not working. Or is there a way of automatic sorting of items in tablenode's section?

Edit, I've just found out, that there's also tableNode.DeleteRow(pos); method, and it looks like it's working...
You've said, that operations directly on Rows collection are not supported right now, that's probably the problem - it might be confusing, because there's no warning about that in VS, only in documentation, but most users first try available methods and only after some problems they're looking into documentation