Lanzelet

Found out, it was the "Kerbal Inventory System (KIS)" Mod. Without it, Startup works fine.

Ok... I just emptied the GameData Folder with all the mods and now it is loading as it should. So it was in fact a mod with my KSP...

Got the same Problem, Loading-Bar is stuck at the beginning with "Loading part upgrades". Got a few Mods in it, but I read in the steam-forums, that the same problem is also with vanilla, also after reinstalling.