My rain barrel pump 4700/2 inox Art. 1766 automatic keeps switching off. What is wrong?

The dry-running protection behaves as follows:
The pump switches off automatically if there is no pumped liquid.
The pump then switches on in a cycle of 30 sec. on - 5 sec. off (4 times).
This cycle is repeated in 1 hour, 5 hours, 24 hours.....
As soon as the pump is at least 4.5 cm in the water again, the pump is ready for operation again.