xml2json.php responds with 500 internal server error

[simevo]

cd fattura-elettronica-json
curl -v -X POST -F 'xml=@samples/IT01234567890_FPA01.xml' https://teamdigitale1.simevo.com/test/paolo/xml2json.php

gets HTTP/1.0 500 Internal Server Error; inspectiing on the server tail /var/log/apache2/error.log reveals:

[Wed Jul 18 14:35:28.429282 2018] [:error] [pid 27263] [client 151.60.7.58:34084] PHP Catchable fatal error:  Argument 1 passed to Simevo\\Xml2Json::__construct() must be an instance of Simevo\\string, string given, called in /var/www/html/test/paolo/xml2json.php on line 16 and defined in /var/www/html/test/paolo/Xml2Json.php on line 49
[Wed Jul 18 14:35:28.429317 2018] [:error] [pid 27263] [client 151.60.7.58:34084] PHP Stack trace:
[Wed Jul 18 14:35:28.429331 2018] [:error] [pid 27263] [client 151.60.7.58:34084] PHP   1. {main}() /var/www/html/test/paolo/xml2json.php:0
[Wed Jul 18 14:35:28.429343 2018] [:error] [pid 27263] [client 151.60.7.58:34084] PHP   2. Simevo\\Xml2Json->__construct() /var/www/html/test/paolo/xml2json.php:16