This has happened to me several times while using it with my N85-3 as of late.
I do NOT have conversations installed so I doubt it has anything to do with that.
What happens is I'll try to login in the Contacts on Ovi application and during the login cycle, I will be ejected from the application and the app closes.
When I try to access Contacts on Ovi again, there's nothing in the tab. (ie. I open contacts, I can see my address book and if I press right (>), I still see my address book. If I press right again, I get into my group list. If I press left (<) from here, I am in the contact's tab but still see my group list. If I press left yet again, I will get back into my address book.
To mitigate this, I have JbakTaskman running so I open it and shut down the contacts application. Then, when I reopen contacts, it lets me get back into the Contacts on Ovi tab.
However, in this screen, the application does not respond to any keystrokes. Even when I open the options menu and select 'SELECT' from there to try to 'Go Online', nothing happens.