Have you changed in editor a default group for lines?
If you want AI buses to use a skin you specified (in Grundorf Hof group) the default group for lines should be Grundorf Hof.
I guess you've changed it to Busses group which is useless because in map you can find only 2 kinds of buses. (according to [ailist])
So there is no need to create new same group with buses (expect AI cars, trains, etc.).
You just threw out some buses so for what you need new group?
For now during gameplay you can see SD85s and D92s driving as AI traffic