Mark, as discussed yesterday, we are having problems with elastic bamboo on unicorn. These problems may well be due to the environment (in particular the outgoing http proxy that everything on unicorn needs to go through), but I think we do need the bamboo team's help to get to the bottom of this. I talked to Przemek last night as you suggested, and he did provide some very valuable input.

I've attached the agent and server logs for 1 failure scenario here:

This happened on zxu.jira-dev.com. I created 3 agents in total. 
1 agent was busy running a build, and the other 2 agents were both idle, 
but a build was still queuing, waiting for an available agent. 

We can see some exceptions in the the server log and agent log of the 2 idle agents.