HOR-1249 Plugin's end points routes aren't generated when disabling/enabling a plugin.

c Fix routes path
c Fix Fix class loader multiple classes
c Fix plugin.singleton serialization

workflow/engine/classes/class.pluginRegistry.php

@@ -169,7 +169,7 @@ class PMPluginRegistry
      */
     public function serializeInstance ()
     {
-        return serialize( self::$instance );
+        return serialize( $this );
     }
 
     /**