Can you check what the path Jenkins is monitoring for that slave, as by default there should be more than 1G free.
Going to http://<yourserver:port>/computer/ will show a summary of all slave and if one is offline due to Diskspace this should be highlighted in the relevant column.
Going to the individual slave page http://<yourserver:port>/computer/<yourslave>, will allow you to find out which path is causing the problem, which might be $jenkins_home, or the java tmp dir.
And the configure link from http://<yourserver:port>/computer/ will take you to the page where you can see the current values and enable/disable them if needed.
Hope this will help you track the issue down.
Chris.