It may be encoding the slash. Try using:
WA_DFP_DownloadFile("WA_DownloadResult1", "/usr/home/vewade".($dl->getColumnVal("DateiPfad",false)) ."", "".($dl->getColumnVal("Dateiname",false)) ."", "".($dl->getColumnVal("Dateiname",false)) ."", false, false, false, "", "", "", "");