I have found the reason. The post of the user Monster at Blenderartist is the explanation.
http://blenderartists.org/forum/showthr ... s-Solution----------------------------
Monster:
"Usually only the texture pixels inside the UV-face (including the edges) are used to draw the rendered face."
"But the texture is not used directly. Before it is used as texture it is processed by the OpenGL mid-map filter. The default is to blur/anti-alias the texture. A side effect of this is that the color of a texture pixel is mixed with the color of it's surrounding pixels."
"On the edges the surrounding pixels are not all part of the UV-face. Therefore the rendered face can get color information from outside the UV-face (e.g. from a black border)."
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The problem is also, that the textures and the uvmap have the same size (picture 1). The uvmap size must be smaller as the texture. (20pixel -> 10 pixel margin all around)
Also first I tried to export overlapped texture tiles with the size 4117, 4097 is the originally size, so as to generate an margin of 10 pixels. But the textures overlapped nevertheless with one pixel.
In the second step, I created manually textures with an margin of 10 pixels in photoshop. (picture 2) I used the neighbor textures or mirrored versions of the destination texture for the margin.
Then I imported the meshes into blender, loaded the manually created textures and adjusted the uvmaps 20pixel smaller as the textures. (picture 3) After this the seams was disappears.
It would be nice if this could be automated in L3DT. Otherwise it would be a lot of work, to do that for all tiles of the texture map, the normal map and the specular map.
Thank you for your help Aaron.
PS: Probably a larger margin is better. Depending on the distance of view the seams was a little bit to see. micah702 recommends 10-20 pixel.