Ha, I just found out why I could not get any straight lines anymore. Whatever edge type I chose, I would always create at least one bend. It turned out that my problem was hidden under the menu point of 'Ansicht' ("view"?). There is an option called "orthogonal edges F10" and I might have activated it unconciously. By turning this option off, I was finally able to draw straight lines again! So, I think the placement of this menu point and its effects on drawing edges is a bit misleading. However, I am happy that I could get rid of this.