I have had that error just once.
I resynchronised the file straight away and it disappeared.
I don't have my tree set up to sync automatically though, do you? (I just sync once at the end of a session or thereabouts.)
Have you tried seeing what happens if you turn off your anti-virus / firewall?
(Not permenantly, just while you try synchronising

)
I do have a quick interent connection, maybe that helps.