No available plugins - Jenkins

I have installed a fresh jenkins on my amazon ec2 micro instance but when I access the plugin manager, I don't have any plugin available. What's wrong?

I used this guide to install https://wiki.jenkins-ci.org/display/JENKINS/Installing+Jenkins+on+Ubuntu

I'm running ubuntu 11.04 64 bits and jenkins-war:1.450

Looking my log file I see

"windmill": {"buildDate": "Feb 06, 2010", "dependencies": [], "developers": [{"developerId": "admc", "name": "Adam Christian"}], "excerpt": "This plugin allows you to configure and run <a href='http://www.getwindmill.com/'>Windmill</a> functional tests.", "labels": ["external"], "name": "windmill", "previousTimestamp": "2009-06-25T20:40:46.00Z", "previousVersion": "1.4", "releaseTimestamp": "2010-02-06T13:59:22.00Z", "requiredCore": "1.319", "scm": "svn.dev.java.net", "sha1": "npNnBKCCpnFCi+Tsw/QA1mQyyyI=", "title": "Windmill Plugin", "url": "http://updates.jenkins-ci.org/download/plugins/windmill/1.5/windmill.hpi", "version": "1.5", "wiki": "https://wiki.jenkins-ci.org/display/JENKINS/Windmill+Plugin"}, "ws-cleanup": {"buildDate": "Dec 07, 2011", "dependencies": [], "developers": [{"developerId": "
    at net.sf.json.util.JSONTokener.syntaxError(JSONTokener.java:512)
    at net.sf.json.util.JSONTokener.nextString(JSONTokener.java:244)
    at net.sf.json.util.JSONTokener.nextValue(JSONTokener.java:352)
    at net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:875)
    at net.sf.json.JSONObject.fromObject(JSONObject.java:170)
    at net.sf.json.util.JSONTokener.nextValue(JSONTokener.java:355)
    at net.sf.json.JSONArray._fromJSONTokener(JSONArray.java:917)
    at net.sf.json.JSONArray.fromObject(JSONArray.java:139)
    at net.sf.json.util.JSONTokener.nextValue(JSONTokener.java:358)
    at net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:875)
    at net.sf.json.JSONObject.fromObject(JSON