This took me a while to figure it out, especially because you didn't post the pageURL!!!
You MUST specify the pageURL.
You MUST specify the SWF player
and you MUST use the -y (playpath) parameter.
Code:
rtmpdump -r "rtmpe://fms-fra21.rtl.de/rtlnow/" -y "mp4:4/V_255543_B3JU_Z27406_80986_h264-mq_22ee4f9b9dbf233c55b7382215155a40.f4v" -f "WIN 11,5,502,146" -W "http://rtl-now.rtl.de/includes/vodplayer.swf" -p "http://rtl-now.rtl.de/csi-vegas/zartbitter.php?container_id=110539&productdetail=1&paytype=ppv&season=12" -o "CSI-Vegas.flv"