I'm not sure where this error is coming from. I thought it was maybe for an old backup/sync app I used called Spideroak, but my limit there is 6gb. I have used Box, Dropbox (still do with over 12gb of free space), Spideroak, Foldersync, and Mozyhome in the past. Spideroak and Dropbox are the only two known to still be active. I only use the free 5GB iCloud backup for iOS device backups. I cannot seem to find where it's coming from.
Also, the Dismiss button does just that, and Upgrade Today does nothing, not even providing a hint (like a tooltip or anything).
Also, Activity Monitor app doesn't appear to provide any hints from active processes either. Figured I should add that in case someone thinks of it, too. It could be something obscurely named, and I'm missing it, but nothing obvious comes to to list.