Create Category within posts screen error

[cfastenrath]

When creating a new category within the "create posts" screen (post.php), there will be a javascript error. The new category will be created, but is only visible after refresh of the page.

It is a completely fresh installation of wordpress. Updated to 4.8.
Only default themes and plugins are activated.

Uncaught TypeError: Cannot read property '0' of undefined
    at Object.c [as addAfter] (load-scripts.php:115)
    at Object.j.is.d.element.wpAjax.validateForm.d.data.match.d.complete (load-scripts.php:10)
    at i (load-scripts.php:2)
    at Object.fireWith (load-scripts.php:2)
    at y (load-scripts.php:4)
    at XMLHttpRequest.c (load-scripts.php:4)

Response from admin-ajax call:

<?xml version='1.0' encoding='UTF-8' standalone='yes'?><wp_ajax><response action='add-category_9'><category id='9' position='-1'><response_data><![CDATA[<li id='category-9'><label class="selectit"><input value="9" type="checkbox" name="post_category[]" id="in-category-9" checked='checked' /> again</label></li>]]></response_data><supplemental><newcat_parent><![CDATA[<select  name='newcategory_parent' id='newcategory_parent' class='postform' >
	<option value='-1'>&mdash; Übergeordnete Kategorie &mdash;</option>
	<option class="level-0" value="9">again</option>
	<option class="level-0" value="1">Allgemein</option>
	<option class="level-0" value="8">Nochmal</option>
	<option class="level-0" value="7">Test</option>
</select>
]]></newcat_parent></supplemental></category></response></wp_ajax>

[cfastenrath]

There were additional lines after the closing php tag in an uploaded file in the plugin folder.
Therefore headers were already sent which results in this error!.