I don't use that provider or have an 810 but a quick look around tells me the settings are standard SIP like most others. Have you tried to delete the account on the N8 and start fresh? Or create a new account and set it up (disabling the other)? Are you using a SIP auth-id under Advanced settings as well as the regular username?

My N800 made the diablo transition and my SIP account against a home brew asterisk box works like it did before, very good and at least as good as a cell phone but much cheaper.