Nah, solved this thingy. The problem was that I had another mod installed which replaced the "EuFictional.bin" file in Assets\Kuju\RailSimulator\TemplateRoutes. This file pointed to the weather settings of the Ruhr-Sieg-Line (The mod can be found
here, it's called "Seebergbahn linked with the weather from Hagen - Siegen"). But because I also had the
Ruhr-Sieg Enhancement Pack by Bermúdez, it replaced the default Ruhr-Sieg-Line weather settings and added the new moon but didn't include the default moon texture in the *.bin files. So I had no moon texture, simple as that. Now reverted back to my backuped "EuFictional.bin" file and all is fine now.
Edit: After thinking about that, I noticed that my explanation doesn't really make sense as the file pointed to the correct weather settings, they were just from another route. So it probably could be that the skydome of the Seebergbahn route is too small for the new big moon from the enhancement pack.