the texture of the cities at night are much too clear and it is a border between the city and the fields much too pronounced

https://drive.google.com/open?id=1_O7fP2_nN5BsTLvYSk0U5fv5O2BM2T4G

https://drive.google.com/open?id=1ZDPjhF1R6n29a1nEi4HhB4f07Nr1d4-8

thanks for your help

You see the limits of landclass based terrain combined with nightlighting that is NOT with dynamic  lightsources.

 

so how can i fix it ?

 

You can't have real dynamic nightlighting, unless you own a CRAY or similar 😎

You can darken the night tiles via the shaders but that wont blend the borders and it will darken ALL tiles, not only the citytiles.

 

yet on the p3d of a friend there is not this ugly border. you can see on the picture https://drive.google.com/open?id=1zFs9mdPbJy5nZI8UXO4KiDgawsl2jTD_

I don't know YOUR system (texture addons like ORBX, nightlighting by Vector or Black Marble, Shader tools etc.) nor that from your friend so its just wild guessing we can do.