exePath must be specified when not running inside a stand alone exe

You need to use a different configuration manager in a web context. The following code
block shows an example of how to deal with this:

System.Configuration.Configuration configuration = null;         
if (System.Web.HttpContext.Current != null)
{
   configuration =
       System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~");
}
else
{
  configuration =
      ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
}