Thanks for the quick responses,
I reinstalled the game as you told me to do, still I am unable to open it via Steam. For some reason, I can now open the game over the Bannerlord.exe file on my D drive ( D:\SteamLibary\steamapps\common\Mount & Blade II Bannerlord\bin\Win64_Shipping_Client ) which was not possible earlier. I dont have much time now to test if the game runs without any problems but I made a battle in one of my campaigns and walked a bit over the map and it seemed like the game was running without any problems. Still the problem with the launcher is there and I was thinking that I may have overseen a folder but I dont think so. I remember to have installed the mod modlib and that there was a button in my laucher which was something like open mod list or so, I dont remember exactly, and when I tried to open it the Launcher was crashing but I could restart it, maybe it has something to do with that. This was shortly before the whole game and launcher were unable to run.