Attached to the buildings there where a lot of spaces and edges.
After I finished my bunker board I got the idea to replace the cardboard base of the roads with MDF tiles.
And so I cutted off the siedewalks and removed the remaining cardboard pieces from the bottom sides...
...and glued the sidewalks onto the MDF tiles. No more warping!
The only problem with this technique is that the MDF tiles are thicker than the cardboard bases and I placed all buildings on such cardboard bases, so the roads are standing higher than the buildings. But this is easily solved by adding some MDF pieces to the bottom of all buildings so they have the same level as the roads. Now I just got to get some more MDF to finish all road tiles and reinforce the building bases.