Sorry you're having problems. Sound like the tokens related to authentication stored on server side and agent side don't match. The agent is succeeding in reaching server. Failure occurs when the server is connecting back to the agent's port 2144.
If you delete an agent's /data directory, and then start it up, that will force the agent to requery you for connection setup details. That might solve your problem. If it doesn't, will you please let me know?
Just to help with completeness, if you have (for other reasons) placed the agent host in the hosts table of the server or identified it to itself in its own /etc/hosts, you may need to alter the *default* for the agent communications IP address to match the hosts table entry rather than 127.0.1.1.
That is what allowed me to make it work on an ubuntu-8.10 server which had those alterations both in the server and the agent host tables.
My guess is that if you are using /etc/hosts the old-fashioned way, to resolve destinations without DNS, then you will encounter this.