How can I fill wall screw holes so I can reposition the screws?
I'm not sure what "solid walls" meant in 1920's Germany but I'd guess it is either brick, concrete or building-blocks (aerated or aggregate blocks of varying density).

In general I've never found a way of filling such holes that would prevent a subsequent overlapping use of a masonry-drill from being deflected towards the old hole.

I'd consider 


enlarge the hole to include the new position, the fill it so that the drill encounters only homogeneous material.
rotate the fitting 180 degrees. 
move it all 1 cm vertically and/or horizontally.
use an intermediate mounting (e.g. a painted wooden plate)
buy a different fitting.