So actually I want to texture a dense 420mio tris model, but for testing regarding the problem here I work with a 40 mio tris model with 60 x 8k textures - this is the optimal unwrap.
All images used for texturing do have a texture-layer now, also the smartphone images! However the smartphone images are not 16bit but 8.
Texturing gets stuck at around 80% preprocessing!
However the problem is with the Smartphone images - when I exclude them from texturing - preprocessing completes.