Issue started after upgrading a full-working server from CentOS 6.3 to CentOS 6.4 (both 32 and 64bit).
User's authorization and authentication are on MS Active Directory.
SSH and sudo work fine but is not possible to get them access on webmin.
Seems to be a problem with user groups from AD: To let my user to log in again, I need to explicit my username in /etc/sudoers.
Working untill CentOS 6.3:
%LinuxAdmin ALL=(ALL) NOPASSWD: ALL
Working on CentOS 6.4:
user.name ALL=(ALL) NOPASSWD: ALL
Only webmin seems to be affected after distro's upgrade.