One way to replicate this gradebook issue:

1. Take a backup of a course from Moodle 1.9 with Turnitin.
2. Restore it in a blank course on the current (2.2.x) server. You will get Turnitin error as expected.
3. Go back to the partially restored course, confirm that gradebook is functional.
4. Take a backup of same Moodle 1.9 course, but this time without Turnitin assignment.
5. Restore it deleting the existing course first.
6. Check gradebook - it is no longer available.

Basically it seems that "Delete the contents of this course and then restore" option does not reset course parameters related to Turnitin, and this in turn can mess up the grades. It seems advisable to reset Turnitin parameters while restoring a course with "Delete the contents of this course and then restore" option. This is just one way of generating the gradebook bug, but other scenarios might be true as well - we had one case when the gradebook was fine after restoring the course and stopped working later on.