Same problem. On my MacBook Pro when I select "Online Only" for a folder or file in Selective Sync, the icon changes to the blue circle with the arrows, to show that it is loading or thinking. Then after that it goes to "Local". I've checked and the Dropbox extension is installed without any problems. I clicked on some large folders to go "Online Only" that had some files that were already "Online Only" and some that weren't. The trouble is they all seem to be going to "Local" and right now the drive is filling up and is close to be critically full.
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