Seems that the HTML editor abstraction is not implemented consistently:
Server Error in '/' Application.
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Unable to cast object of type 'NopSolutions.NopCommerce.Web.Controls.NopHTMLEditor' to type 'AjaxControlToolkit.HTMLEditor.Editor'.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.InvalidCastException: Unable to cast object of type 'NopSolutions.NopCommerce.Web.Controls.NopHTMLEditor' to type 'AjaxControlToolkit.HTMLEditor.Editor'.
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace:
[InvalidCastException: Unable to cast object of type 'NopSolutions.NopCommerce.Web.Controls.NopHTMLEditor' to type 'AjaxControlToolkit.HTMLEditor.Editor'.]
NopSolutions.NopCommerce.Web.Administration.Modules.ProductInfoControl.rptrLanguageDivs_ItemDataBound(Object sender, RepeaterItemEventArgs e) +385
System.Web.UI.WebControls.RepeaterItemEventHandler.Invoke(Object sender, RepeaterItemEventArgs e) +0
System.Web.UI.WebControls.Repeater.OnItemDataBound(RepeaterItemEventArgs e) +115
System.Web.UI.WebControls.Repeater.CreateItem(Int32 itemIndex, ListItemType itemType, Boolean dataBind, Object dataItem) +138
System.Web.UI.WebControls.Repeater.CreateControlHierarchy(Boolean useDataSource) +478
System.Web.UI.WebControls.Repeater.OnDataBinding(EventArgs e) +61
System.Web.UI.WebControls.Repeater.DataBind() +76
NopSolutions.NopCommerce.Web.Administration.Modules.ProductInfoControl.BindData() +207
NopSolutions.NopCommerce.Web.Administration.Modules.ProductInfoControl.Page_Load(Object sender, EventArgs e) +93
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
System.Web.UI.Control.OnLoad(EventArgs e) +91
System.Web.UI.Control.LoadRecursive() +74
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2207
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Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.1