When I fixed a bug that did not permit the default route to be in a sub-folder, I introducted a different bug (#5285), which this should fix
diff --git a/system/libraries/Router.php b/system/libraries/Router.php
index f192379..cf42516 100644
--- a/system/libraries/Router.php
+++ b/system/libraries/Router.php
@@ -98,7 +98,22 @@
 				show_error("Unable to determine what should be displayed. A default route has not been specified in the routing file.");

 			}

 

-			$this->uri->uri_string = $this->default_controller;

+			// Turn the default route into an array.  We explode it in the event that

+			// the controller is located in a subfolder

+			$segments = $this->_validate_request(explode('/', $this->default_controller));

+

+			// Set the class and method

+			$this->set_class($segments[0]);

+			$this->set_method('index');

+			

+			// Assign the segments to the URI class

+			$this->uri->rsegments = $segments;

+			

+			// re-index the routed segments array so it starts with 1 rather than 0

+			$this->uri->_reindex_segments();

+			

+			log_message('debug', "No URI present. Default controller set.");

+			return;

 		}

 		unset($this->routes['default_controller']);