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As part of a private modification, I edited a few o3d model files in Blender using the "Blender-O3D-IO" plugin. I did not touch the textures of the models in the program. However, after exporting, in OMSI in daylight, the textures of the edited models appear darker than the others. In evening light conditions, they are all equally dark, so it works well in the dark. What could be the reason for this?
set the specular of the blender material to zero
try export x and then the OMSI X Converter
setting the specular material to 0 did not solve the problem.
the blender directx export addon not working above Blender version 2.79 - the o3d file I edited cannot be imported into version 2.79. 
In the meantime, using this exporter addon solved the problem.