OK, I solved this problem:
Formerly I just installed the DSC Run Time System from the CD. Strangely all things work a little bit without special error messages. Now I create an installer and included to install the DSC/SV/Runtime thingies. Now I get the expected behaviour for datasocket read.
But why oh why was there the impression DS would work correctly? I'd expect that ds read would fail utterly if not all needed components are installed.
And why do I need to install additional components if I install the LV DSC RTS completely?