WordPress Plugin Issues

Answer 1

Edited: I found my original answer was somewhat correct. Below are the details needs to fix your problem.

Change your WP_Widget_Excert function to the following

function WP_Widget_Excerpt() {
        $widget_ops = array('classname' => 'WP_Widget_Excerpt', 'description' => __('Text or HTML'));
        $control_ops = array('width' => 400, 'height' => 350);
        $this->WP_Widget('WP_Widget_Excerpt', __('Multi Excerpt'), $widget_ops, $control_ops);
}

Answer 2

If your Widget is doing something simple you can use Widgetifyr.com to create your widget for you. I creates the old style widget as well as the new 2.8+ class based widget. This way your widget will run on more versions of Wordpress.