Once I upgraded to Jenkins 1.5, my slave agent ("mac-mini") could no longer connect. Here's an example session (I changed the hostname and credentials to protect the guilty):
I then used Charles Proxy to capture the HTTP traffic to see what was going on. I've attached a file of that capture, but the relevant problem is here:
The problem is that the slave is being told to connect via HTTP, but (thanks to our proxy) Jenkins is only available via HTTPS. I checked my configuration and it is correct. I believe the issue is caused by this change on January 15th. The protocol of the request from Apache to Jenkins is HTTP, however the client must use HTTPS as I configured our root URL.