Problem:

HTML code should be placed inside a XML document, but the template converts (escapes) the data to XML.

<?xml version="1.0" encoding="UTF-8"?>
<productRequest>
<product>
<subtitle>TEST &amp; Co</subtitle> <!-- OK -->
<productDescription><![CDATA[&lt;br&gt:Aus Aluminium in trendiger Farbgebung. &lt;br&gt;]]></productDescription> <!-- Not OK -->
</product>
</productRequest>

Solution:

Use the template raw() function to prevent syntax escaping:

<subtitle>[% Field1.0 %]</subtitle>
 <productDescription><![CDATA[[% Field2.0.raw() %]]]></productDescription>
 <?xml version="1.0" encoding="UTF-8"?>
 <productRequest>
 <product>
 <subtitle>TEST &amp; Co</subtitle> <!-- OK -->
 <productDescription><![CDATA[<br>Aus Aluminium in trendiger Farbgebung. <br>]]></productDescription> <!-- OK -->
 </product>
 </productRequest>