Hi Christos,
I would check you are using the latest version of the Turnitin Plagiarism plugin. Take a look at the notes on this release;
https://moodle.org/plugins/plagiarism_turnitin/3.5/25137
There is also a tracker issue on their github which could be related;
https://github.com/turnitin/moodle-plagiarism_turnitin/issues/604