It is because the request string don't have compact=1
in it.
Most tracker require that nowadays. The legacy way is too ineffective.
See BEP 23: Tracker Returns Compact Peer Lists http://www.bittorrent.org/beps/bep_0023.html
Try:
http://torrent.ubuntu.com:6969/announce?
info_hash=%9b%db%bbI%f0%85%a2%d1%5d%96%ac%fa%bf%f81%06%001O%e0
&peer_id=ABCDABCDABCDABCDABCD&port=6882&downloaded=0
&uploaded=0
&left=0
&event=started
&compact=1
ie
http://torrent.ubuntu.com:6969/announce?info_hash=%9b%db%bbI%f0%85%a2%d1%5d%96%ac%fa%bf%f81%06%001O%e0&peer_id=ABCDABCDABCDABCDABCD&port=6882&downloaded=0&uploaded=0&left=0&event=started&compact=1
and the answer from the tracker is:
d8:completei4e10:incompletei0e8:intervali1800e5:peers6:******e