It looks like this error is coming from WidgetZone.cs in the App_Code\Controls folder. Around line 230.
Inside that catch() handler, you could add a call to have the details logged, like:
That should record the details in logger.txt in the App_Data folder. Or you could directly output the details into the message you are already seeing...
lit.Text += ex.StackTrace;
You might have to dig into the InnerException too, if there is one.