try the following command. if it doesn't work provide the verbose output. it would be better if you can provide working proxy or vpn because stream is geo blocked.
Code:
rtmpdump -r "rtmpte://flash.streampower.be/festivals_2k11_pu/_definst_" -a "festivals_2k11_pu/_definst_" -p "http://www.skynet.be/generation-nl/muziek/festivals/vod/1/36/10/4501" -W "http://static.generation.portal.skynet.be/swf/player5.6.swf?v=20110607" -y "mp4:graspop_2k11/avenged_sevenfold.mp4" --start 0 --verbose -o Test.flv