thank you Xerxes
that's how I'm changing the textures,....just without a new name. I prefer to save the original textures in a separate folder (I'm just to lazy to apply the small textures which have a "wrong" name  by hand :) )

& thank you RHaseltine,...
I do report every bug, when it's possible. But this time I got no failure message or log-file that I could report. DS just did nothing (well...nothing except silently working hard in the background without telling me, as I learned now g).
I guess I just overloaded my system with too much work (it's just a lazy office laptop, that isn't supposed to work as a render-machine, I think)
But thanks for the explanation of what is happening during that "optimization" !!
I never realized that step before...

& thanks again to all for your help & information!