FWIW, I'm having the same problem. Using a Dell XPS 410 desktop wired and an HP laptop wireless with a Canon inkjet printer attached to the 655 USB port. Both computers running Vista Home Premium with latest updates. The problem occurs on both, even after turning off the Shareport update feature. I've also gone the route of uninstall - reinstall. I don't use Windows firewall, I use Comodo firewall but I've tried completely turning off the firewall and it still occurs.
I've worked around it by always exiting the Shareport utility after I use it so that only one computer is connected at a time and even then, only when needed. But if I forget to exit, the same popup 'Connection failed,...' message invariably reoccurs - sometimes within minutes, sometimes several hours or even a day later.
This is too good a feature of the Dir-655 to not get fixed.