After the clutch and everything else was assembled, all noises were gone, I wanted to start a test drive. Believe it or not, yet another problem occured. I was unable to to find reverse...
Today I removed the shift rod coupler cover and found this:
I tried some deforming of the metal, but I finally used the grinder:
This cut out is invisble, after the cover is re-installed. I have no idea why the gearbox sits like this, but this was the only way to solve it on short notice!