Moving a website's home directory off the network disk breaks PHP

Hi everyone. I’ve got an example of some really bizarre behaviour here.

A few months ago, we had Virtualmin happily running on a single server. Then we got interested in high availability after a major hardware failure, and we set up a network disk cluster with Gluster, bought Cloudmin, and then used Virtualmin’s “change home directory” feature to move all our hosted websites to that disk cluster.

Everything works fine now.

But when I try to move one of those sites back to the local disk for performance reasons, I only see the PHP code displayed in the browser. Every other site works completely normally. I see nothing unusual in the Apache configuration in /etc/apache2/sites-enabled/, and really the only difference is that the paths have all changed. I’m completely lost here.

We’re using Cloudmin 8.4 Pro and Virtualmin 5.01 GPL on Ubuntu 14.04.

This appears to be an Apache issue, since if I change the DocumentRoot folder to the new location manually, the problem comes up there too.