Since updating to 1.5.12 of the time-stamper plugin the live-update of the console output is broken for me. Downgrading to 1.5.11 fixes the problem. I supected a core change but discussion on the jenkins-user list gave the right clue to try to downgrade the timestamper plugin.

In this discussion another user reported the following firebug error, maybe that's helpful:

Firebug reports this error after receiving a successful POST from http://

/logText/progressiveHtml :
TypeError: container is null
http://jenkins-url/static/4a3b5c01/scripts/prototype.js
Line 1798