Yesterday, I have found a thread about spark plug lubrication. Based on the conversation there, I'm thinking that I may caused the starting problem.
To prevent stuck, I applied a little copper paste on the plug threads that might cause inappropriate ground for the plugs.
Afaik copper paste is conductor, so this is just another theory.
I drive about 50km in city yesterday, with a few cold and hot start, and no problem at all with the startup.
I'm also curious about the consumption with the new plugs, 60km from last tank fill, and the gauge is still at full. I know it's not a linear scale, though. My Mondeo had full gas gauge after 120km with a low fuel warning at about 480km...