There are the following workarounds for this issue:
Either install original version of Webmin from http://www.webmin.com/.
Or execute the following commands after installation of the BitDefender Webmin module:
# cd /usr/local/share/webmin
# ln -s ../../../share/webmin/web-lib.pl
# cp /usr/local/share/webmin/BitDefender-2.0/config /etc/webmin/BitDefender-2.0/config