I recently downgraded from Pro to GPL using the apt-get instructions here: https://www.virtualmin.com/documentation/installation/uninstalling
It worked fine and the dashboard reported that it wasn't Pro anymore. Everything seems to be working the same as before.
Since then I've added two new virtual servers and both show a 500 Internal Server Error when accessing PHP files on those domains, while the other domains added before the downgrade are fine. If I access an HTML file, it is served with no problem. The PHP is not only PHP that is working fine on other domains on the same server, it won't even echo out a simple Hello World from test files.
The Apache Error Log shows this for the 500 error:
[Sat Aug 04 20:27:35.982751 2018] [fcgid:warn] [pid 668] (104)Connection reset by peer: [client MYIPADDRESS:47963] mod_fcgid: error reading data from FastCGI server
[Sat Aug 04 20:27:35.983009 2018] [core:error] [pid 668] [client MYIPADDRESS:47963] End of script output before headers: index.php
I've looked at this error and done several things that have fixed this error for other virtualmin/Apache users with no success. Nothing on this page has worked: https://www.virtualmin.com/node/25539
I don't really want to reinstall Virtualmin and bring everything back from backup, as I don't want to bring whatever configuration error to a new setup if I can help it plus I have some pain in the rear dependencies to set up for one domain. Does anyone have any other troubleshooting steps to try?