Not immediately seeing why this was not working after looking at the source code for the text editor user control and dnn label control I had to try replicating your problem in a DNN source installation to be able to step into the label control code. Here's the actual exception that is being logged (twice for each aspx page load) in the event viewer (with the key point highlighted):
StackTrace:
Message: DotNetNuke.Services.Exceptions.ModuleLoadException: The control collection cannot be modified during DataBind, Init, Load, PreRender or Unload phases. ---> System.Web.HttpException: The control collection cannot be modified during DataBind, Init, Load, PreRender or Unload phases. at System.Web.UI.ControlCollection.Add(Control child) at DotNetNuke.UI.Utilities.ClientAPI.RegisterDNNVariableControl(Control objParent) at DotNetNuke.UI.Utilities.ClientAPI.get_ClientVariableControl(Page objPage) at DotNetNuke.UI.Utilities.ClientAPI.GetClientVariableList(Page objPage) at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientVariable(Page objPage, String strVar, String strValue, Boolean blnOverwrite) at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientReference(Page objPage, ClientNamespaceReferences eRef) at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientReference(Page objPage, ClientNamespaceReferences eRef) at DotNetNuke.UI.Utilities.DNNClientAPI.EnableMinMax(Control objButton, Control objContent, Int32 intModuleId, Boolean blnDefaultMin, String strMinIconLoc, String strMaxIconLoc, MinMaxPersistanceType ePersistanceType, Int32 intAnimationFrames, String strPersonalizationNamingCtr, String strPersonalizationKey) in C:\DNN5Dev\Library\UI\Utilities\ClientAPI.vb:line 155 at DotNetNuke.UI.Utilities.DNNClientAPI.EnableMinMax(Control objButton, Control objContent, Int32 intModuleId, Boolean blnDefaultMin, String strMinIconLoc, String strMaxIconLoc, MinMaxPersistanceType ePersistanceType, Int32 intAnimationFrames) in C:\DNN5Dev\Library\UI\Utilities\ClientAPI.vb:line 149 at DotNetNuke.UI.Utilities.DNNClientAPI.EnableMinMax(Control objButton, Control objContent, Int32 intModuleId, Boolean blnDefaultMin, String strMinIconLoc, String strMaxIconLoc, MinMaxPersistanceType ePersistanceType) in C:\DNN5Dev\Library\UI\Utilities\ClientAPI.vb:line 141 at DotNetNuke.UI.Utilities.DNNClientAPI.EnableMinMax(Control objButton, Control objContent, Boolean blnDefaultMin, MinMaxPersistanceType ePersistanceType) in C:\DNN5Dev\Library\UI\Utilities\ClientAPI.vb:line 111 at DotNetNuke.UI.UserControls.LabelControl.Page_Load(Object sender, EventArgs e) in C:\DNN5Dev\Library\UI\UserControls\LabelControl.vb:line 243
The RegisterClientVariable method requires that the page's form contain a hidden input control with id and name of "__dnnVariable". Add the following markup to your aspx page's form and I think you'll have this working:
<input id="__dnnVariable" runat="server" name="__dnnVariable" type="hidden" />
No doubt you may find other similar problems when hosting dnn controls outside of a module container on the Default.aspx page.