jens38:
I will try to get now a zener diode 18V to do some tests, and check whether I can see a change to the todays performance.
An 18v zener may be enough, but higher than 18v would be better, if the Vds rating of the mosfet allows (mosfet Vds rating must be at least the supply voltage + the zener voltage + 1v for the diode). The reverse current rating of the zener needs to be at least as high as the current in the relay coil, which is around 170mA if it is 2W @ 12v.
jens38:
I will give this one also a trial. Advantage: Probably I have some resistors in the range of 10 Ohms available.
10 ohms is much too low. The relay coil resistance (from 2W @ 12v) is around 72 ohms and the resistor needs to be at least as high as that to make much difference. See the calculation I gave for max resistance in my earlier post.