Ok ... I replaced the shockwave_alpha.png and now the models are not disappearing any more. Really great ... thanks again.

Unfortunately it's not fixing the TargetMap issue. I hope it's not a bug in my project that overwrites the render_target pointer in some way.
But then again the render_target should be updated with every frame, shouldn't it. So I guess it would lead into an engine crash if something would write into its address.