The subject matter is happening on a fresh Virtualmin setup. The website pages display ok in Apache mod_php and FPM modes, however switching to FCGId (which is set by default) makes PHP pages to download.
Checking the Virtualmin configuration shows everything is alright:
The status of your system is being checked to ensure that all enabled features are available, that the mail server is properly configured, and that quotas are active ..
Your system has 992.58 MB of memory, which is at or above the Virtualmin recommended minimum of 256 MB.
BIND DNS server is installed, and the system is configured to use it.
Mail server Postfix is installed and configured.
Postfix is configured to support per-domain outgoing IP addresses.
Apache is installed.
The following PHP versions are available : 5.4.16 (/bin/php-cgi), 5.4 (mod_php)
PHP-FPM support is available on this system.
PHP versions have changed to 5, 5.4 since last check. Regenerating any missing php.ini files.
Webalizer is installed.
Apache is configured to host SSL websites.
MySQL is installed and running.
ProFTPD is installed.
Logrotate is installed.
SpamAssassin and Procmail are installed and configured for use.
ClamAV is installed and running.
Plugin DAV Login is installed OK.
Plugin AWstats reporting is installed OK.
Plugin Protected web directories is installed OK.
Using network interface eth0 for virtual IPs.
Default IPv4 address for virtual servers is xx.xx.xx.xx.
Default IP address is set to xx.xx.xx.xx, which matches the detected external address.
Both user and group quotas are enabled for home and email directories.
All commands needed to create and restore backups are installed.
The selected package management and update systems are installed OK.
Chroot jails are available on this system
.. your system is ready for use by Virtualmin.
Updating all Webmin users with new settings..
Updating status collection job ..
Applying web server configuration ..
Re-loading Webmin ..