If the message you get is akin to the following
(or perhaps “deleted card”) then you are facing the problem that you still refer to some question/card somewhere, while that question/card has been deleted in the meantime.
The easiest way to solve it, is to completely expand all cards and all change sets, and check where orange selection boxes appear. You can do so by clicking on the expansion arrows while both Shift and Alt (Option on Mac) are held down. For example:
will show the following, indicating that the placeholder still happens to refer to some deleted question:
Tip: you can prevent such errors from happening by selecting a question (or card), and then checking the contents of the Dependencies pane in the Q&A. In the example scenario above, the question that was deleted, actually showed the following before it got deleted:
