3rd party firewalls, internet security software, etc can all cause troubles with sharing folders.
Note that even with some firewalls and internet security softwares disabling them will not totally disable them. And even with some (bad) ones you need to totally uninstall them to lift the restrictions caused by them. However, most have options to allow (ignore) certain programs, ports, etc to be used without restrictions.
But all this said, recently more people have reported problems with the sharing folders not being able to sync (I also have experienced the same yesterday and today btw). But this is due server problems and you can't do anything about it than wait and try again later.
PS: Don't double post. If you need to add something before someone has replied, edit your message using the button underneath your post. Also don't bump threads if they don't contain posts older than a few days, this isn't an instant chat program or irc client.