I have a nokia E52 and in general i'm very happy with the Nokia Ovi Suite software. But today it refused to synchronise my calendar with a "Sync Cancelled" error message, other items synchronised fine. The solution was to reset the nokia profile/files on my computer. Make sure to close Outlook, Ovi Suite, and end the "nokiamserver.exe" process. Then open up explorer and browse to C:\Users\Name\AppData\Local and C:\Users\Tim\AppData\Roaming. You could delete these files, but just in case i renamed the "Nokia" and "Nokia Ovi Suite" folders to "xNokia" and "xNokia Ovi Suite". If anything goes wrong you can always rename them back to their original names.
Then open up Ovi Suite again and reconnect the phone. It should synchronise fine then, there must be some bug with the calendar synchronisation that can corrupt the local database.