The problem was that the rendertargets were cleared with the sky color at the begin of a view render cycle, just like the normal screen. I found that the DirectX Clear function affects all color targets, not only COLOR0.

I'll mention this in the manual. Simple solution: Either don't clear them,

vec_set(sky_color,vector(0,0,0));

or disable the render targets with a view event when the view should not write into them.