I think that you are right,
I have studied the resonator and the schematic again and the resonator I have only has two legs in stead of 3.
So I think that the person who ordered these parts for me made a little mistake. I think he has ordered the ZTA-20MX in stead of the ZTT-20MX
http://dkc3.digikey.com/PDF/T073/P1287.pdf
This resonator has no connection to the ground.
It looks reasonable that this causes the problem.
On the attached overlay I wil have to connect the resonator to the left and right hole of X1?