I have worked in saw mills. They usually have a mechanical safety stop that keeps the two halves of the machine from coming together when working between the two halves. The cylinders must be retracted in a safe direction and then the pressure is off the hooks so they can be lifted. I have seen many presses but most rely too much on electronics. I would like to see more mechanical stops or hocks that extend and block the press and the mechanical stops need to be withdrawn. It seems to me a spring would do the job and the spring could be forced back with air when everything is working right.
As far as valves go. Again in the saw mill industry there are often blocking valves between the servo valve and the cylinder. These valves must be energized open. There are contacts and relays that must keep these valves energized open. If any relay or contact opens the valves close. However, this doesn't keep the cylinder from moving it just keeps the cylinder from moving quickly. The cylinder will probably drift slowly due to leakage. This is a problem because most of the time the press is applying force on the cap or blind side of the piston. As the pressure equalizes between the rod and cap side the cap side force will be higher and extend the rod. On top of that gravity is pulling the press down.
Also, check the electrical output to the valve. Make sure it can't possibly fail high ( energized ) or there is some way to detect when it does.