I asked the same question the other day, and the answer I got was that there was already a suggested edit in the queue. When you then go to submit your edit, the queue rejects it as there is already a suggestion pending.
It is a horrible error message, though. It doesn't accurately explain the actual error happening. It must be that the programmers lock a queue by removing the permission to edit the question in the same way that people with very low reputation points can't edit a question. I imagine you'd get the same error if you had a new user try to edit a question (if they could somehow bypass the missing 'edit' button).