Reproduce: index.php:

require_once 'Zend/Form.php';
$form = new Zend_Form(array(
    'elements' => array(
        'submit' => 'submit'
    )
));
$form->addElementPrefixPath('My_Decorator', './decorators/', 'decorator');
$form->submit->setDecorators(array('My'));

and create an empty file "./decorators/My.php".

This will trigger the error "Fatal error: Class 'Zend_Loader_PluginLoader_Exception' not found in [...]Zend\Loader\PluginLoader.php on line 316".

Suggested solution: Add the following line to Zend\Loader\PluginLoader.php at line 316.

                        Zend_Loader::loadClass('Zend_Loader_PluginLoader_Exception');