I found out that the vertical beams of my Ender 3 were not perpendicular to the base in the y-direction. I have designed these simple z-braces which can be added without further modification of the printer. I have designed them in such way that the threaded rod is as close to the frame as possible to make it really stiff.
Keep in mind that the maximum z-height is reduced to approximately 220mm! This is not an issue for me because I will never print this high and if I do, I’ll use my CR-10.