The original bug is http://community.thoughtworks.com/posts/5724dc11b1,
The problem is when anti-virus application installed on agent side, the agent always think it's a new agent, so need to approve it again. it's a bug in the current release, and we'll fix it in next release.
The workaround is change the setting in your antivirus software, for example in ESET NOD32:
Antivirus Settings -> Web access protection -> Http, https
disable http checking or remove the port 80 from the list