Excel 2013 connection not open error

[masplin]

I'm experimenting with Excel 2013 and keep running into the same problem. Things are going fine and creating new measures etc then suddenly I get this "connection not open" error. I close the workbook reload, same error. I restore a previous working version from dropbox, same error. Only solution is to get an old 2010 version and reconvert it. This is making it unworkable due to lack of backwards compatibility. I'm guessing this is structural and not actually due to the workbook or a previous version would work. Very frustrated. Does anyone know what this error means please?

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Error Message:
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The connection is not open.

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Call Stack:
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at Microsoft.AnalysisServices.AdomdClient.AdomdUtils.CheckConnectionOpened(AdomdConnection connection)
at Microsoft.AnalysisServices.AdomdClient.AdomdCommand.CheckCanExecute()
at Microsoft.AnalysisServices.AdomdClient.AdomdCommand.ExecuteReader(CommandBehavior behavior)
at Microsoft.AnalysisServices.BackEnd.DataModelingView.<>c__DisplayClassb.<ExecuteCommand>b__7()
at Microsoft.AnalysisServices.BackEnd.DataModelingConnectionRetry.ExecuteOperation[T](DataModelingSandboxConnection connection, Boolean retryable, ExecuteRetryCode code)
at Microsoft.AnalysisServices.BackEnd.DataModelingView.ExecuteCommand(String commandText, CommandParameters parameters)
at Microsoft.AnalysisServices.BackEnd.DataModelingView.LoadValues(Int64 rowNum)
at Microsoft.AnalysisServices.BackEnd.DataModelingView.GetCellValue(Int64 rowIndex, Int32 columnIndex, Boolean cacheOnly)
at Microsoft.AnalysisServices.Common.SandboxEditor.UpdateFormulaBarFromTableWidget()
at Microsoft.AnalysisServices.Common.SandboxEditor.sheetsTab_sheetChanged(Object sender, EventArgs e)
at Microsoft.AnalysisServices.Common.SheetTabs.SetSelectedSheetInternal(String key)
at Microsoft.AnalysisServices.Common.SheetTabs.PopulateSheetListUI(String keyOfNewSheetToSelect, Boolean toRecreateUIElements)
at Microsoft.AnalysisServices.Common.SandboxEditor.RefreshSandboxEditor(String tableId)
at Microsoft.AnalysisServices.XLHost.Modeler.ClientWindow.RefreshClientWindow(String tableName)

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