Without being there : DOWN.

The black box is likely a buoancy device/swimmer.

Water flows in, box get's "pressed" upwards. When the water raises to a certain level the box is so far up that it closes the valve. Usually over some sort of lever mechanism.

Yours isn't closing the valve completely, so water keeps flowing in and then leaves through the overflow.

Possible causes

• The box lost it's buoancy, if it is a container, it has a hole or something like that. UNLIKELY, because it more or less wouldn't work at all.

• The box lost the right position. These things can come lose and then aren't closing the valve early enough. This can be adjusted by moving it in reference to the lever system (your up/down). By moving it down, it will "swim" earlier and close the valve earlier. LIKELY cause.

• Defect O ring, the inflow valve of course contains sealants, O rings. Maybe the lever pushes it shut like it should, but still a tiny amount of water comes in. POSSIBLE you would need to change the seals or even the inner mechanism of the water box.

• The mechanism got scaled, over time a crust of minerals accumulated and prevents the valve from closing. Give a good amount of citric or vinegar acid in the water tank and let it fill (Or special anti scaling stuff) POSSIBLE

Easiest way to figure out what might be the issue:

Empty the water tank(flush) and when it keeps filling see if raising to he black box /swimmer by hand closes the valve at some point. Then you verified that all seals work and scaling is not an issue. If it doesn't close no matter if the swimmer goes up then the issue is with the valve.