Contax 645 & Kodak 645c Pro Back problem

[ican]

Just bought mint Kodak 645c back - everything seems ok and back even turns on and shows menus... But attached to Contax 645 it fails to take pictures. Camera got stuck - no viewfinder indications, no shutter release, no autofocus. When film back is attached everything goes normal.
Nothing seems broken or dirty (i mean electrical contacts). Does anyone know what prevents camera from taking pictures? Please help.