As noted you should check out /var/log/fai/grml/last/ and if you
can't find the source of the problem on your own please provide full
logs instead of snippets, see http://grml.org/grml-live/#how-to-debug

$ tar zcf grml_live_problem.tar.gz /etc/grml/grml-live.conf /etc/grml/grml_live.cmdline /etc/grml/grml-buildd.conf /var/log/fai /etc/grml/fai
tar: Removing leading `/' from member names
tar: /etc/grml/grml_live.cmdline: Cannot stat: No such file or directory
tar: /etc/grml/grml-buildd.conf: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors

$ tar zcf grml_live_problem.tar.gz /etc/grml/grml-live.conf /var/log/fai /etc/grml/fai