Right-Click the City of Heroes application, select Show Package Contents, and then navigate to Contents > Resources > transgaming > drive_c > coh (or that last part might be Program Files > City Of Heroes).
Look for a file called updater.lock and throw it away.
It should work after that.
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I'm getting a really funny error. It says Updater error "A copy of COH updater is already running" although I know it's not. I've already restarted the mac and I'm still getting this. The only thing I can think to do now is to reinstall the software. Does anyone out there have a quick fix?