I found finally alone the problem, some security plugin like Sucuri Security allow you to protect wp-content folder.
Sucuri Security plugin generate .htaccess in the wp-content with :
deny from all
That was the problem !
I resolve it with an other .htaccess in wp-content/plugins/simple-ads-manager/ where i put :
<FilesMatch « sam-ajax.php|sam-ajax-loader.php »>
Allow from all
I was thinking about desactive this plugin to test, but protection .htaccess was not clear when you uninstall or delete the plugin.
I hope my messages will help someone else.