Warning (2) : SimpleXMLElement::__construct(): Entity: line 190: parser error : PCDATA invalid Char value 11 [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): <title>test strange character in city field</title> [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): ^ [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): Entity: line 194: parser error : PCDATA invalid Char value 11 [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): <city> University Park, PA</city> [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): ^ [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): Entity: line 200: parser error : PCDATA invalid Char value 11 [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): escription><p>here the same strange character appears in the description: [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (2) : SimpleXMLElement::__construct(): ^ [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Utility/Xml.php, line 298]
Warning (512) : Unable to emit headers. Headers sent in file=/chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php line=37 [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 65]
Warning (2) : Cannot modify header information - headers already sent by (output started at /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 159]
Warning (2) : Cannot modify header information - headers already sent by (output started at /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /chroot/home/niles/test.mathmeetings.net/conflist5/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 192]
Error: Serialization of View data failed.

Serialization of View data failed. 📋

Cake\View\Exception\SerializationFailureException
CORE/src/View/SerializedView.php at line 82 (edit)
        if ($serialize !== false) {
            try {
                return $this->_serialize($serialize);
            } catch (Exception | TypeError $e) {
                throw new SerializationFailureException(
                    'Serialization of View data failed.',
                    null,
                    $e,
                );
Caused by String could not be parsed as XML Exception
CORE/src/Utility/Xml.php at line 298 (edit)
        self::_fromArray($dom, $dom, $input, $options['format']);
        $options['return'] = strtolower($options['return']);
        if ($options['return'] === 'simplexml' || $options['return'] === 'simplexmlelement') {
            return new SimpleXMLElement((string)$dom->saveXML());
        }
        return $dom;
    }

If you want to customize this error message, create templates/Error/error500.php