There was an issue earlier today affecting Turnitin submissions.
Turnitin Support have advised:
Some integration users may have experienced intermittent problems with TurnitinUK between 10:30 and 14:20 GMT. System is normal now.— Turnitin Status (@TurnitinStatus) October 29, 2014
(See the right hand column on the front page of Moodle for their most recent updates.)