I had a nagging issue with Dropbox on one of my Windows 8 machines (and persisted through Windows 10 after upgrading). For some reason, it worked initially then stopped working citing permissions issues as the reason. I tried several fixes from web searches as well as fixes suggested by Dropbox support, however, nothing worked. Until this. I found a thread stating that several people had run Windows Repair by Tweaking.com and had success. I don't particurlarly running these sorts of tools but I was at my wits end with Dropbox not working so I gave it a shot and it resolved the issue. Dropbox has been syncing away ever since.