The vacuum cleaner has a water switch. If the switch is out of the water, the vacuum cleaner switches off. Therefore, a minimum water depth of approx. 50 cm is required. If the appliance is running without water, this switch is defective.

This suction cup has a slide switch on the front with a wheel screw to fix it.
This fixation may no longer be correct. Loose or even lost.
Slide it to position 1 and then fix it with a little glue.
The suction cup only switches on when it is completely immersed in water and switches off again shortly after it is removed.
switches off again shortly after removal.

It has a float contact and only works completely under water when the handle is pointing upwards. Perhaps the float is dirty.