When opening the plug-in:
ERROR: require_once(): Failed opening required '/opt/psa/admin/plib/modules/transip/library/Form/../../vendor/autoload.php' (include_path='/opt/psa/admin/plib/vendor/plesk/zf1/library:/opt/psa/admin/plib') (Settings.php:29)
Seems like the plug-in is no longer maintained.