Hi Blackstridaaustria,
thanks for your comments.
The washer I use is 1,5mm thick but I don’t really get your point. Why is it necessary to adjust the outer surface of the cup spring with a recessed washer? For my understanding you will compensate the thickness of the washer by just inserting the screw a little bit less. As you write above the tension of the cup spring is the relevant parameter that will be assured by the correct fixing torque of the bolt no matter how many washers you add (as long as the minimum insertion depth of the bolt is assured).
I thought the function of the recess of your washer was just to keep the cup spring properly in place since the “armour washer” results in a flat surface at the outer side of the frame without any edge. But since I noticed that my stainless steel washer on top of the cup spring is already slightly deformed to a bowl-like shape it does this job perfectly. 
I perfectly agree with you that the TiO2 grease does not work properly since the bolt still tightens and unlooses during unfolding and folding of the bike.
So I will order Loctite and an anti seezing compound to finish the job.