I asked the same [question][1] the other day, and the [answer][2] 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).

[1]: https://android.meta.stackexchange.com/questions/1511/what-does-this-error-mean-this-edit-would-have-to-be-a-suggested-edit
[2]: https://android.meta.stackexchange.com/a/1512/11190