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>/…
I usually avoid using Webmin, but I had to set it up for a company I worked for.
The guys there were not familiar with Linux and they needed an interface to keep configuring the Postfix server I settled (with Spamassassin as an antispam and Amavis as content filtering).
I had to face a problem while I was trying to get the Postfix Webmin module working.