any hints on how to get rid of the seam that comes with the symetrical lattice

You can't, it's always been an issue in Bryce, you can get close using smoothing and clipping but there will always be a seam, sometimes you can kinda hide it with textures though.

The higher the terrain mesh resolution you use, the smaller the seam appears. If you are making hard edged objects with straight lines you can sometimes "boolean" away the seam by grouping it with a negative cube. Clay is right though, the seam is always there to some extent.

You can try exporting the lattice/terrain and re-importing it into Bryce, and then smoothing the mesh. Unfortunately, this almost always smooths it too much deforming the terrain/lattice, and the imported mesh is always bigger (file size) than the original...but in some circumstances it can work out. Might be worth a shot, lol.