Thanks for your help! So far lowering the grafic settings did the trick. (And soon I will get a new and better laptop. Yay!)

While playing on a bigger (+100 hours played) and slightly modded singleplayer save, the game (seemingly) randomly crashed, noting "Unexpected error occurred". It might has happend just before an autosave - the dialog for the autosave hasn't shown, but there is a save file with "tmp" in it's name ...