After tracing execution of the code I narrowed the cause down to the apparent non-setting of httpd_modules{‘mod_suexec’} but couldn’t pin down why.
Incidental to this tracing, in Webmin, I accessed
Servers] Apache Web Server] Re-Configure Known Modules
(to ensure mod_suexec was seen as enabled).
On leaving the page, to be sure the options were set, I pressed the “Configure” button although I hadn’t altered any settings.
I tried the Virtualmin configuration check again but this time it complained that “mod_actions” wasn’t enabled!
I returned to
Servers] Apache Web Server] Re-Configure Known Modules
and ticked "mod_actions" and the pressed the "Configure" button.
Returning to Virtualmin I again tried to do a check and this time to my surprise it passed the Apache check but complained about Webalizer not being configured or installed.
I pressed the Virtualmin "Module Config" link at the top-left of the page and then disabled Webalizer:
Webalizer report generation enabled? "No"
On returning to Virtualmin it succeeded in doing a check and now presents the Virtualmin Virtual Servers, master admin mode, page.
Checking the enabled modules once again at
Servers] Apache Web Server] Re-Configure Known Modules
it still shows "mod_action" un-ticked but Virtualmin appears to be working!
As I didn’t restart apache2 after ticking “mod_actions” I’m wondering if Webmin reloaded/restarted apache in the background (it didn’t say it had) or just changed a Webmin option setting, that Virtualmin subsequently found pleasing.
The upshot is that - somehow - Virtualmin has sprung to life!