It seems that you have added a custom control on the pages or inside a page template that you are using for the problematic pages. The name of the control is LeftMenu:
SitefinityWebApp.Custom.LeftMenu.LeftMenu.GetFirstNode(SiteMapNode currentNode) +56
SitefinityWebApp.Custom.LeftMenu.LeftMenu.GetFirstNode(PageSiteNode currentNode) +243
To resolve the error remove the custom control and review it.
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