Help Cylinder head seal ????

I think that my ZKD has said goodbye. Symtoms: Cooling water loss, light smell in warm engine after … something? The breakdown service does not find, however. The engine reaches normal temperature, but does not get warmer than 90 degrees. The cooling hoses are not harder than normal, so it does not build up any pressure. The interior is dry and under the car it is never na ss. Of course I replenished water and aired the system because I thought that there was air in the system somewhere and that’s why it now needs water, because I was quite careless until ́dato with the refilling of the cooling liquid. Was necessary up to now, because always full. today I filled the balancer quite fully and drove about 17 km and now it is half empty. Maybe one of you still has an idea what it could be or what I can still check about the cause definitely to recognize? What could it be different? there are other causes w for coolant loss. I am grateful for every tip and help. LG Oneling