I've started a case with qlik support earlier today, will update this thread when I have something to share.
I believe that the issue is that the import task tries to remove a user that is used as a recipient for a report/task, and that's why the task execution throws a fk violation.
I have the same error message but my fk violarion is on table smtp_destination. How many tasks do you have with a cc recipeint? Go through all tasks and remove the recipients on cc one by one and try running the import task in between would be my suggestion .
On my case, the issue goes away if I unmark the "remove users.." checkbox on the import task .Try removing that setting if you need your task to suceed.