The problem was found in the CT TBB (Thanks to @ChrisSummers).
The XSLT CT TBB outputting this component had a reference to an XSLT mediator HTMLEncoding helper:HtmlDecode() method, which is fine for normal text, but not for XSLT. I removed it and issue solved:
<xsl:stylesheet version="1.0" xmlns:tcm="http://www.tridion.com/ContentManager/5.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:tridion="http://www.tridion.com/ContentDelivery/5.3/TCDL" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:helper="http://www.tridion.com/xslthelper" xmlns:systeemcode="http://www.indivirtual.nl/SysteemCode" xmlns:xhtml="http://www.w3.org/1999/xhtml" exclude-result-prefixes="tcm xsl tridion xlink systeemcode xslthelper helper xhtml">
<xsl:output method="text" omit-xml-declaration="yes" indent="yes" encoding="utf-8" standalone="no"/>
<xsl:variable name="content" select="/tcm:Component/tcm:Data/tcm:Content/systeemcode:SysteemCode"/>
<xsl:template match="/">
<xsl:value-of disable-output-escaping="yes" select="$content/systeemcode:code"/>
</xsl:template>
</xsl:stylesheet>