In my case, Dropbox overlays were starting with a ” (quoted identifier) in the registry. I deleted all the ” prefixes and restarted explorer.exe.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\ShellIconOverlayIdentifiers
Edit: I installed Windows 10 and this solution didn’t work for me. So I just went to the same registry location and deleted all Google and SkyDrive records and restarted explorer.exe.
Second edit: After installing TortoiseGit it fixed everything without any customisation.