I recently upgraded from version 7 to version 15.1.2 (NuGet) of the AjaxControlToolkit. I've noted several issues with the HtmlEditor: (all issues tested on the latest stable Chrome release.)
1) Required CSS references aren't present when inheriting the control into a custom class, e.g., public class MyEditor : AjaxControlToolkit.HtmlEditor.Editor { ... } is missing most styling when used on a page. Changing the class to inherit Control and adding an Editor to this.Controls programatically works as expected.
2) The "Insert Link" popup is missing all text, and doesn't actually insert a link into the content window after use.
3) The paste button throws a JavaScript error: "Uncaught Sys.ArgumentUndefinedException: Sys.ArgumentUndefinedException: Value cannot be undefined.
Parameter name: format" (paste from Word might have a similar issue, I'm not entirely sure.)
All other default toolbar buttons seem to work correctly.
Comments: Some code for #2 (though it's probably not going to help you, since there's literally nothing fancy going on) Default.aspx: ``` <%@ Page Language="C#" CodeFile="Editor.aspx.cs" Inherits="_Default" %> <%@ Register TagPrefix="ajaxToolkit" Namespace="AjaxControlToolkit.HtmlEditor" Assembly="AjaxControlToolkit, Version=15.1.2.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e" %> <!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml"> <head runat="server"> <title></title> </head> <body> <form id="form1" runat="server"> <asp:ScriptManager runat="server" ID="scriptManager"></asp:ScriptManager> <ajaxToolkit:Editor runat="server" ID="editor" Width="960px" Height="360px"></ajaxToolkit:Editor> </form> </body> </html> ``` Default.aspx.cs ``` using System.Web.UI; public partial class _Default : Page { } ``` Result:  Project is a blank Web Site (not a Web Application) with AjaxControlToolkit added via NuGet.
1) Required CSS references aren't present when inheriting the control into a custom class, e.g., public class MyEditor : AjaxControlToolkit.HtmlEditor.Editor { ... } is missing most styling when used on a page. Changing the class to inherit Control and adding an Editor to this.Controls programatically works as expected.
2) The "Insert Link" popup is missing all text, and doesn't actually insert a link into the content window after use.
3) The paste button throws a JavaScript error: "Uncaught Sys.ArgumentUndefinedException: Sys.ArgumentUndefinedException: Value cannot be undefined.
Parameter name: format" (paste from Word might have a similar issue, I'm not entirely sure.)
All other default toolbar buttons seem to work correctly.
Comments: Some code for #2 (though it's probably not going to help you, since there's literally nothing fancy going on) Default.aspx: ``` <%@ Page Language="C#" CodeFile="Editor.aspx.cs" Inherits="_Default" %> <%@ Register TagPrefix="ajaxToolkit" Namespace="AjaxControlToolkit.HtmlEditor" Assembly="AjaxControlToolkit, Version=15.1.2.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e" %> <!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml"> <head runat="server"> <title></title> </head> <body> <form id="form1" runat="server"> <asp:ScriptManager runat="server" ID="scriptManager"></asp:ScriptManager> <ajaxToolkit:Editor runat="server" ID="editor" Width="960px" Height="360px"></ajaxToolkit:Editor> </form> </body> </html> ``` Default.aspx.cs ``` using System.Web.UI; public partial class _Default : Page { } ``` Result:  Project is a blank Web Site (not a Web Application) with AjaxControlToolkit added via NuGet.