[TYPO3-german] Problem mit Install-Tool nach Update von 7.6 auf 8.7

[Klug]

Hallo,
auf einer Testumgebung mit XAMPP 7.1.4 versuche ich ein Update von Typo3 7.6.18 auf 8.7.1. 
Nachdem ich die Dateien der neuen Version in das Typo3 Verzeichnis kopiert habe erscheint beim ersten aufruf des Install-Tools wie gewohnt die Aufforderung zur Erstellung der Datei ENABLE_INSTALL_TOOL. 
Nachdem ich diese erstellt habe erscheint beim erneuten Aufruf des Install-Tools jedoch nicht die gewohnte Eingabemaske sondern der folgende reine Text:

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no"> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <link rel="stylesheet" type="text/css" href="../Resources/Public/Css/install.css?1497735689" /> <link rel="icon" type="image/vnd.microsoft.icon" href="../Resources/Public/Icons/favicon.ico?1497735689" /> <script type="text/javascript" src="../../core/Resources/Public/JavaScript/Contrib/jquery/jquery-3.2.1.min.js?1497735689"></script> <script type="text/javascript" src="../../backend/Resources/Public/JavaScript/jquery.clearable.js?1497735689"></script> <script type="text/javascript" src="../Resources/Public/JavaScript/bootstrap.min.js?1497735689"></script> <script type="text/javascript" src="../Resources/Public/JavaScript/chosen.jquery.min.js?1497735689"></script> <script type="text/javascript" src="../Resources/Public/JavaScript/Install.js?1497735689"></script> 

<div class="container"> <div class="page-header"> <h1 class="logo-pageheader"><img src="../Resources/Public/Images/typo3_orange.svg" width="130" class="logo" /> Site: Typo3 8.7 Testsite <small>Login to TYPO3 8.7.1 Install Tool</small></h1> </div> <div class="row"> </div> <div id="t3-install-box-body"> <h3></h3> <form method="post" class="form-inline" id="t3-install-form-login"> <input type="hidden" name="install[controller]" value="common" /> <input type="hidden" name="install[action]" value="login" /> <input type="hidden" name="install[token]" value="e0f1e46ecd27f0cbcc97b1eca4a553559f97ca43" /> <input type="hidden" name="install[context]" value="standalone" /> <div class="form-group"> <label for="t3-install-form-password">Password</label> <input id="t3-install-form-password" type="password" name="install[values][password]" class="t3-install-form-input-text form-control" autofocus="autofocus" /> </div> <button class="btn btn-default btn-success" type="submit">Login</button> <input type="hidden" name="install[set][login]" value="submit" /> <a href="Install.php?install[action]=logout&install[context]=standalone&install[controller]=tool" class="btn btn-danger pull-right"><i class="fa fa-lock"></i> Lock Install Tool again</a> </form> </div> <div id="t3-install-box-border-bottom"> </div> <div class="panel panel-info"> <div class="panel-heading"><h3 class="panel-title">Information</h3></div> <div class="panel-body"> By default the install tool password is the one specified during the installation. </div> </div> <div class="panel panel-warning"> <div class="panel-heading"><h3 class="panel-title">Important</h3></div> <div class="panel-body"> If you don't know the current password, you can set a new one by setting the value of $GLOBALS['TYPO3_CONF_VARS']['BE']['installToolPassword'] in typo3conf/LocalConfiguration.php to the hash value of the password you desire, which will be shown if you enter the desired password in this form and submit it. <br /><br /> This password gives an attacker full control over your instance if cracked. It should be strong (include lower and upper case characters, special characters and numbers) and at least eight characters long. </div> </div> </div> 

In der gleichen Testumgebung konnte ich ein frisches Typo3 8.7.1 problemlos installieren. Was läuft hier beim Update schief? Wer kann mir weiterhelfen?

Grüße
Franz-Karl