by David Woods.
Thanks Dawn,
I also tried to resubmit the assignment directly through the Tii web interface but got the same message, so the problem was not on the Moodle side.
Your suggestion worked. Of course, the idea is that the student can overwrite his own submission up until the due date; doing so does not result in a high plagiarism match with the earlier submission. Turnitin automatically filters out the earlier submission. But for some reason it wouldn't work (error 413).
Cheers,
Dave