Thank you very much to All and especially to Xwheelguy. The Video brought the solution.
Here was my mistake: When I opened the Vis... Material/Texture only the upper menu shows up - but for a nanosecond - the bigger menu as well (Materials to part) After I tried again and again I figured out it is hidden in the left frame Colum fare below the tree window. It was the last button. You have to scroll with the arrows totally down to the button with the green cuboid and the sphere, connected with 2 black perpendicular lines. When you click this with the right Mouse button the words "un-dog" shows up. I clicked it, deleted the decal and I went to the fridge and drunk a beer in 10 seconds. The fight lasts almost 2 days long.
Thanks again to all!!! From Bogdan