If you work with Webmin on Debian, you may have some problems when you download a module which is not available in a package (i.e, Amavis).

When you try to access to it, it may fail and give back an error message like :
Undefined subroutine &main::init_config called at /usr/local/share/webmin/<your_module>/…

This is just because the module was made to work in a Linux environnement like Red Hat derived distros.

Debian uses some particular paths, so you have to :

$ mv /usr/local/share/webmin/squidguard/ /usr/share/webmin/
$ cd /usr/local/share/
$ rmdir webmin
$ ln -s /usr/share/webmin