I would think it has to do with the url rewriter. Probably losing the context of the page.
based on another forum post I saw cutechat post this C# code to handle that problem.
override protected void OnLoad(EventArgs args)
{
base.OnLoad(args);
Context.Items[typeof(CuteWebUI.UploadModule)]=true;
}
Quick change that is definetely worth a shot, give it a try.