Object already open

Here's a newbie question that I may find the answer to but haven't yet. When I try to edit a cue list in the editor I get the message "The object is already open via another playback - this can result in unexpected stage looks whilst editing"

I've released everything I can think of and closed all the windows, but I still get the message. I detached all the cuelists from masters. (Actually, there are only two cue lists so far and the channels in them are mutually exclusive at this point.) When I look at the output window, everything is 0% and nothing is sourcing anything.

So, what is "the object" that is open? The cuelist I'm editing? Another cuelist? The programmer? A channel? A parameter? A cue? How can I find out what it is? I know you experts all understand what "objects" can be, but for the inexperienced, object is a rather generic term that could include just about anything, thus this message is incredibly obtuse.
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