You say that, when you release the button, the contact opens again (resistance goes to infinity). But the light stays on, right? That means there must be some device which has a "memory" of the last action: you push a button, the light goes on and stays on, until you press the button again (or another button). I'm sure there is a device in the cabinet (Dutch: "schakelkast") which toggles its output on every pulse it receives, and which in this way controls the lamp.
The wire on the top right of the switch also hints in this direction; it's a signal wire, it's too thin to be a power wire. My hunch is that it carries the pulses to the actual switching device (I don't know what it's called).

Occupancy sensors, timers, dimmers, and other "smart" switches often are required to be independently powered. If you look at this diagram from the devices documentation (PDF), you'll see that there are three ways this requirement is achieved.

Neutral Wire Required

The first method, is to simply require a neural wire. In this configuration, the device draws power using the ungrounded (hot) conductor and grounded (neutral) conductor. It also has a separate switched conductor, that it uses to control the load.

This setup would be wired like this...

Minimum Load Required

This method draws power using the ungrounded (hot) conductor, and the switched conductor. So the device is actually in line with the load.

This setup is wired like this...

Ground Wire Required

This method draws power using the ungrounded (hot) conductor, and the grounding conductor. It's wired similar to the Neutral Required devices, however, it uses the grounding conductor instead of the grounded conductor. This means that there will be a small amount of current on the grounding conductor, and that the grounding conductor is required for the device to operate.

This setup would be wired like this...

Notice there's a bare, and green wire connected to ground in this diagram.

tl;dr

Your device

The device you're using (MS-OPS5M-XX) requires a ground to operate, according to the documentation.

Solutions

Install grounding conductors

One solution, would be to install a grounding conductor with this circuit. This will likely require quite a bit of work, and might be quite costly.

Install a grounded conductor

It may be possible to extend a grounded (neutral) conductor from the light to the switch box, which could then be used to power the device. In this case you'd have to purchase a different device (one that requires a neutral rather than a ground).