Thanks for your reply. I just checked the model and it has only 8 vertice for sure!

My theory: A vertex can have only 1 uv coordinate. But the uv mapping created with "create 6 side mapping" needs at least 3 vertices per "real" vertex because otherwise it won't be possible to split the sides of the cube as shown in the skin editor screenshot. But I think the 2 additional vertices are somehow hided by the engine and not returned through ent_getvertex(). Thus I can only change one of those vertices which leads to the strange behavior. What do you think?