Ok, it seems there's an issue with simpole-fields: they use a mix of wordpress jquery and google jquery. Never going to work reliably.
The solution is simple: replace the function wp_deregister_script in simple-fields.php by this code:
wp_deregister_script('jquery');
wp_deregister_script('jquery');
wp_deregister_script("jquery-ui-core");
wp_deregister_script("jquery-ui-sortable");
wp_register_script('jquery', 'http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js', false, '1.4.2');
wp_register_script('jquery-ui-core','http://jquery-ui.googlecode.com/svn/tags/latest/ui/minified/jquery.ui.core.min.js',false,'1.8');
wp_enqueue_script("jquery-ui-core");
wp_register_script('jquery-ui-sortable','http://jquery-ui.googlecode.com/svn/tags/latest/ui/minified/jquery.ui.sortable.min.js',false,'1.8');
wp_enqueue_script("jquery-ui-sortable");
wp_enqueue_script("jquery-ui-effects-core", "http://jquery-ui.googlecode.com/svn/tags/latest/ui/minified/jquery.effects.core.min.js");
wp_enqueue_script("jquery-ui-effects-highlight", "http://jquery-ui.googlecode.com/svn/tags/latest/ui/minified/jquery.effects.highlight.min.js");
wp_enqueue_script("thickbox");
wp_enqueue_style("thickbox");
define( "EASY_FIELDS_FILE", menu_page_url("simple-fields-options", false) );
}
And then it all works ... maybe something to give to the author of the simple-fields plugin.