Hi:

ok you can delete this broken user entering in the system as root and then running kuser, or a different user management application, then you can create a new user and use it.

I also recomend you to do:

# apt-get update
# apt-get upgrade
# apt-get dist-upgrade

In order to correct other problems you can find. (Ok run it in konsole and you can run the just the last one, I think it will dowload about 300MB so be patient and wait for your updated knoppix debian)

It would work fine.

I also think you don't have to delete the default user neccesarily but I have to investigate if changing the initial scripts....

I hope to be helpful