TurnItIn had advised that this issue was affecting all TurnItIn services worldwide, including access to the service, assignment submissions, the feedback studio, and similarity report processing.
From Turnitin:
We encountered a disruption between the following times: 14:52 – 07:06 GMT Feb 15-16, 2019. We’re pleased to announce that this has been resolved. Please accept our apologies if you were impacted by the disruption.