Update to 6.0 causes XML Parsing Errors

Just upgraded the form and now my theme – that works with XML parser – crashes. I get the error

[27-Jun-2015 04:40:21 UTC] PHP Warning:  SimpleXMLElement::__construct(): Entity: line 130: parser error : Opening and ending tag mismatch: wphead line 1 and script in /storage/web/public/sites/www.site.nl/wp-content/themes/theme-name/vihv/control/TControl.php on line 91

The line it refers to is having this function:

function GetXml() {
			try {
				$Xml = new SimpleXmlElement(TXml::MakeTree($this->GetData(), get_class($this)));
				return $Xml->asXml();
			} catch(Exception $e) {
				//print_r($this->GetData());
			}
		return "<".get_class($this)."/>";
		}

When I deactivate the plugin all is well, but then I will of course no longer have the beatiful forms I am using. There must be a HML tag not being closed properly or an additional one missing. Will need to see how you add html to figure this one out..
https://ww.wp.xz.cn/plugins/custom-contact-forms/