Surprisingly the same problem appeared.
It turned out to be the no return valve in the outlet that caused the problem.
To much backpressure for the pump to get up in speed.

With the valve out, both pumps worked well.
But the returning water from the hose was enough to trip the pump to restar.
You get the picture?
The pump was cycling "forever".   

Even if we raised the level sensor the same happened.