I have two projects, Project A and Project B. Project A has existed for a couple of years and works as it should. We are expanding and therefore needed to create project B. Since Project B should have the same settings as Project A, we copied Project A and called it Project B. It worked as it was supposed to for several months. Suddenly the field for "due date" was missing. It is not impossible to add a due date to any new task as the field is gone, no matter what type of issues are being created. It is not possible to add a due date after the issues have been created either.
It is impossible to edit due date in the "List" or Issue" view either.

The project settings are the exact same for Projects A and B. Everything in Project A is still working perfectly. I have checked issue types, workflows, screen schemes, and project settings for issues. They share every setting I can find. Any tips?