Could be terrible wrong and apologize for that but I think there are nested forms inside com_joomlaupdate default view.

In the file administrator/components/com_joomlaupdate/views/default/tmpl/default.php

line 37~40
<?php if ($this->showUploadAndUpdate) : ?>
<?php echo JHtml::_('bootstrap.startTabSet', 'joomlaupdate-tabs', array('active' => 'online-update')); ?>
<?php echo JHtml::_('bootstrap.addTab', 'joomlaupdate-tabs', 'online-update', JText::_('COM_JOOMLAUPDATE_VIEW_DEFAULT_TAB_ONLINE')); ?>
<?php endif; ?>

think should be between line 34 ~ 35
<div id="joomlaupdate-wrapper">
// think should be placed here
<form enctype="multipart/form-data" action="index.php" method="post" id="adminForm" class="form-horizontal">

Thank you