Hi, I am sorry for not answering earlier.
I am currently accessing the database with MySQL.
From config.ini: $CFG->dbtype = 'mysqli';
As for the modification, we have done some on the webform and my former colleague has done most of the modification - therefore I cannot be so sure if there has been any on the plugin itself. But it shouldn't be any there. Where will I find the plugin files for this module?
Due to the unknown changes, I am also unsure if I can safely upgrade the Moodle core without loosing content.
Perhaps a core update will do - although I am not sure if that will really help if the issue is due to any modification in the past or the database itself.
Thanks for your time!