I`ve created my own test module (containing a view, edit & settings usercontrol) to eliminate some possibilities.

The result was the same, as I described in the previous post.

In the settings.ascx.cs, I added no code behind, just inherited "ModuleSettingsBase".

On the settings.ascx page, I only added the editor, like so:
   <%@ Control language="c#" Inherits="Mvsd.DNN.Modules.MVSDMODULE.Settings" CodeBehind="settings.ascx.cs" AutoEventWireup="false" %>
   <%@ Register TagPrefix="dnn" TagName="TextEditor" Src="~/controls/TextEditor.ascx"%>
   <dnn:texteditor id="teDescription" runat="server" width="450" height="200"></dnn:texteditor>