I have already tried changing the layer grid options in the Layout Settings and enabling the "Disable tiled raster layer exports" Setting in the Export Menu.
In the screenshot, you can see how the brighter Zone of the map does not have these seams. The Heightmap in the Background is a merged product of NASA Satellite Tiles. Could this have something to do with that?
Just South of the Rectangle saying "Hessen" you can see the seams I am talking about.