Are you using the DNN 3.1?

 

if so, please change your web.config to

 

 

<add name="CEHtmlEditorProvider" type="DotNetNuke.HtmlEditor.CEHtmlEditorProvider, DotNetNuke.CEHtmlEditorProvider"
                              providerPath="~\Providers\HtmlEditorProviders\CEHtmlEditorProvider\CuteSoft_Client\CuteEditor\"
                              UseDNNRootImageDirectory="true"
                              RemoveServerNamesFromUrl="true"
                            EnableStripScriptTags="true"
                              UseRelativeLinks="false"
                              RenderRichDropDown="true"
                              UseSimpleAmpersand="false"
                              CustomCulture=""
                              ThemeType="Office2003"
                              DisableAutoFormatting="false"
                              BreakElement="div"
                              EditorBodyStyle=""
                             
                              Admin_AutoConfigure="Full"
                              Admin_SecurityPolicyFile="admin.config"
                              Admin_TemplateItemList=""
                              Admin_DisableItemList=""
                              Admin_AllowPasteHtml="true"
                              Admin_EditorOnPaste="default"
                              Admin_ReadOnly="false"
                              Admin_ShowBottomBar="true"
                              Admin_ShowHtmlMode="true"
                              Admin_ShowPreviewMode="true"
                              Admin_EnableStripScriptTags="false"
                              Admin_EnableBrowserContextMenu="true"
                              Admin_EnableContextMenu="true"
                              Admin_EnableContextMenuEditing="true"
                              Admin_EnableContextMenuFormat="true"
                              Admin_EnableContextMenuInsert="true"
                              Admin_EnableContextMenuInsertAdvanced="true"
                              Admin_EnableContextMenuInsertFiles="true"
                              Admin_EnableContextMenuInsertForms="true"
                              Admin_EnableContextMenuRelative="true"
                              Admin_EnableContextMenuTags="true"
                              Admin_EnableContextMenuVerbs="true"
                                                           
                              Forum_Admins_AutoConfigure="Full_noform"
                              Forum_Admins_SecurityPolicyFile="admin.config"
                              Forum_Admins_TemplateItemList=""
                              Forum_Admins_DisableItemList=""
                              Forum_Admins_AllowPasteHtml="true"
                              Forum_Admins_EditorOnPaste="default"
                              Forum_Admins_ReadOnly="false"
                              Forum_Admins_ShowBottomBar="true"
                              Forum_Admins_ShowHtmlMode="true"
                              Forum_Admins_ShowPreviewMode="true"
                              Forum_Admins_EnableStripScriptTags="false"
                              Forum_Admins_EnableContextMenuEditing="true"
                              Forum_Admins_EnableContextMenuFormat="true"
                              Forum_Admins_EnableContextMenuInsert="true"
                              Forum_Admins_EnableContextMenuInsertAdvanced="true"
                              Forum_Admins_EnableContextMenuInsertFiles="true"
                              Forum_Admins_EnableContextMenuInsertForms="true"
                              Forum_Admins_EnableContextMenuRelative="true"
                              Forum_Admins_EnableContextMenuTags="true"
                              Forum_Admins_EnableContextMenuVerbs="true"
                             
                              Registered_AutoConfigure="Simple"
                              Registered_SecurityPolicyFile="guest.config"
                              Registered_TemplateItemList=""
                              Registered_DisableItemList=""
                              Registered_AllowPasteHtml="false"
                              Registered_EditorOnPaste="text"
                              Registered_ReadOnly="false"
                              Registered_ShowBottomBar="true"
                              Registered_ShowHtmlMode="false"
                              Registered_ShowPreviewMode="true"
                              Registered_EnableStripScriptTags="true"
                              Registered_EnableContextMenuEditing="False"
                              Registered_EnableContextMenuFormat="False"
                              Registered_EnableContextMenuInsert="False"
                              Registered_EnableContextMenuInsertAdvanced="False"
                              Registered_EnableContextMenuInsertFiles="False"
                              Registered_EnableContextMenuInsertForms="False"
                              Registered_EnableContextMenuRelative="False"
                              Registered_EnableContextMenuTags="False"
                              Registered_EnableContextMenuVerbs="False"
                             
                              Guest_AutoConfigure="Minimal"
                              Guest_SecurityPolicyFile="guest.config"
                              Guest_TemplateItemList=""
                              Guest_DisableItemList=""
                              Guest_AllowPasteHtml="true"
                              Guest_EditorOnPaste="default"
                              Guest_ReadOnly="false"
                              Guest_ShowBottomBar="true"
                              Guest_ShowHtmlMode="true"
                              Guest_ShowPreviewMode="true"
                              Guest_EnableStripScriptTags="false"
                        />
                  </providers>