I got this error on about 5 of my reports when upgrading to reporting 2010.1. The errors were on the resx file of the report.
System.InvalidOperationException: Item named 'detail.GridSize' of type 'Telerik.Reporting.Drawing.SizeU' cannot be added to the resource file because it is not serializable. at System.Resources.ResXDataNode..ctor(String name, Object value) at System.Resources.ResXResourceWriter.AddDataRow(String elementName, String name, Object value) at System.Resources.ResXResourceWriter.AddMetadata(String name, Object value) at Telerik.Reporting.UpgradeWizard.Engine.Utilities.ResourceFile.SaveAs(String fileName) at Telerik.Reporting.UpgradeWizard.Engine.ResourceItem.UpgradeInternal() at Telerik.Reporting.UpgradeWizard.Engine.UpgradeItem.Upgrade()
System.InvalidOperationException: Item named 'detail.GridSize' of type 'Telerik.Reporting.Drawing.SizeU' cannot be added to the resource file because it is not serializable. at System.Resources.ResXDataNode..ctor(String name, Object value) at System.Resources.ResXResourceWriter.AddDataRow(String elementName, String name, Object value) at System.Resources.ResXResourceWriter.AddMetadata(String name, Object value) at Telerik.Reporting.UpgradeWizard.Engine.Utilities.ResourceFile.SaveAs(String fileName) at Telerik.Reporting.UpgradeWizard.Engine.ResourceItem.UpgradeInternal() at Telerik.Reporting.UpgradeWizard.Engine.UpgradeItem.Upgrade()