3D Tilemap: lines between Tiles/Chunks

Hey, i made a 3D-tilemap, which works perfectly. The problem is, that you can see these tiny flickering lines where one tiles is colliding with another. I even made a mesh combiner, to combine multiple tiles to chunks. But then i see these lines between the chunks.

I already tried overlapping the tiles, which made it even worse.

How can i fix these lines? Thank you,

michidk