i've been having problems also exporting to .3ds from v.12, and i'm on a quadcore pc running windows xp pro;
i don't get error/bug message but the export process commences (the progress indicator is visible and gets to about 25 - 30% complete) and archicad just 'hangs' (aka not responding) requiring a manual task ending and restarting of software.
i think the culprit is library parts in the model, particularly those generating lots of faces/polygons (e.g. 3d trees/vehicles etc.) but i haven't been able to confirm this.
Of course, every so often, usually when i'm trying to crash it deliberately to analyse the problem it works perfectly.