Now I got it, very tricky [:D]
I played a little bit with your model with regard to customizing textures and additional questions came up. Hope you can answer them:
1. case:
Loading your model, marking all boxes and change texture of the selection (not with the material change panel as being inactive) and got this. Again some areas of distortion:
2. Case
following your advice exporting you model as obj-file and importing it again. Using texture change (not the material change panel) and got this.
Other areas of distortion:
3. case
Doing the same as in case 2, but now using the material change panel and assigning each of the 6 areas the same texture. Result is strange again, because the scaling of the texture was changed (to very small). You see as well a simple wall with the same texture and 100% scaling.
I find this very strange and I don't get the logic behind. Maybe you can shed some light on this. Many thanks.