I got this error when I upgraded an install from v6.1.1 to v6.1.6. Although the offcial instructions state that only the bin folder needs to be updated this is far from being the case. You also need to ensure you use the latest files under /Umbraco and also /Umbraco_client. If you check the namespaces in /Umbraco/Controls/Images, for example you'll see it now lives in a different namespace
'<%@ Control Language="C#" AutoEventWireup="True" CodeBehind="ImageViewer.ascx.cs" Inherits="Umbraco.Web.UI.Umbraco.Controls.Images ... %>
to the original:
'<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="ImageViewer.ascx.cs" Inherits="umbraco.controls.Images.ImageViewer" %>'
I've been working with Umbraco for just over six months and in that time I've yet to see a document from them that is accurate or complete. FFS.