Nobody but root can login to Virtualmin


I am experiencing a major issue with Virtualmin, one that may have existed for a while but that I have only now been made aware of.

Basically, the gist of it is that only the root user can log into Virtualmin. No virtual server owner can log into Virtualmin / Webmin at all. Every time someone tries, they get the typical "Login failed. Please try again" error message.

I checked /var/log/secure but Webmin only reports the failed username and the IP address that made the attempt, but no other information.

Currently all virtual server owners are LDAP users, while of course root is a local user on the system.

These users can login just fine through FTP and SSH, but as stated cannot login at all to Virtualmin/Webmin.

How can I resolve this issue? I am running CentOS 7 with Webmin 1.740 and Virtualmin 4.15 Pro.