This is how we Load User controls into a content place holder.
ctl = (UserControl)LoadControl(PagePath.ReturnPagePath(Request.QueryString["CurrentPage"]));
catch (Exception ex)
Response.Write("Unable to load " + (string)Session["CurrentPage"] + " control");
You need to replace the whole ctl call with the UserControl object the you created. When you Response.Write your execption on a load fail. change the output to whatever you would like to display.
By looking at your naming scheme it looks like you just want to fill a Panel. For this we usually make a Panel on the page and goto the solution explorer then drag and drop the usercontrol we wish to display into the panel. If you are not wanting to change this dynamically later in your solution the drag and drop is the easiest way to go.
It's not a bug, it's a feature.