First time I activated the plugin, everything was working fine. But then some other plugin crashed, so I had to deactivate all the plugins. When I tried to activate the Simple Page Ordering plugin again I got the following error message:
Fatal error: Cannot redeclare class Simple_Page_Ordering in /home/......./www/wp-content/plugins/simple-page-ordering/simple-page-ordering.php on line 12
I tried to reinstalled the plugin (twice), but it didn't change anything.