(Hope I'm doing the right thing, posting it as an issue and not in the forum, I'm not used to this)
On a production debian Wheezy server, LAMP, mysql 5.5.57-0+deb7u1-log, PHP 5.4.45, virtualmin stable and all, everything works fine with mod_fcgid as php execution mode. But if I switch to FPM, in Virtualmin's "Website Options" panel, the next time I reload the website for which I made the change, I have a blank error 500 in my web browser, and in Apache's error log:
[Tue Sep 19 22:01:34 2017] [warn] proxy: No protocol handler was valid for the URL /index.php. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.
Having read https://www.virtualmin.com/node/12107 , I made sure that, Webmin -> Servers -> Apache Webserver -> Global Configuration -> Configure Apache Modules , both proxy and proxy_http are checked: they were already checked.
I went as far as restarting the server, same issue. Switching back to mod_fcgid resolves the issue.
I thought PHP-FMP had left beta status, it's why I am reporting it in the Issues section. I don't ask for support (not like I need PHP-FPM at all costs). Instead, I was hoping a bug report would help. Once again, I hope it's not a bother. If it is indeed an unplanned bug, if you ever want more info, sure, otherwise, no worries.