Better check if the 127.0.0.1:10000 should be working on a box local for Virtualmin, don’t know someone experience with this? ( with your GUI)
Maybe this isn’t suposed to work, or need extra’s , loopback adress is special !!!.. (socket)
Wen doing things local network i alway’s use my own local subnet ip’s but not this 127.0.0.1 , yea i know you can use as , localhost itself , but i don’t like it. 
And hosts file for name’s to ip’s local.
With cache i mean opcache phpfpm combinations as example or other configs like that.
And webbrowsers use sessions like caching things on their own often
LOGS are so important here!
If things in virtualmin are ( hard) coded to use “localhost” somewhere or even domainname, hostname there could be problem then if you use also other ip’s ( in setup) then a internal resolving to the 127.0.0.1. Partly also look in the hosts file. httpd.conf and named,bind and so on
127.0.0.1 is not always the same ip as names are used and resolving local
Could be even a internal conflict internal aplications used to use localhost and 127.0.0.1 combi with the virtualmin interface on port 10000 and browser then, but don’t know about that
Not related maybe but here you have a better english explanation for my first reply here writing about yes no https: and using localhost , ip problems
https://bugs.webkit.org/show_bug.cgi?id=171934#c96
IN VM kinds of configs / installations then better not using the loopback ip , better have routable ip’s then
So maybe the problem are different separate problems and one is that you can’s use att all 127.0.0.1 :10000 and putting things on the wrong path to go for looking for solution for the basic problem
As here ?
Offtopic but i had some webmin virtualmin things related with hsts:
For others not knowing this is related, but if you use some browsers they expect https and if HSTS configured is on now, sometimes the first redirect is not known by the browser and seems not working , try it manually if a https is there next time then browser remember it . (www.nonwww and so on)
(i.e. the first time the request is made, before HSTS can override the request)
I also think newer installations default Virtualmin / Webmin are on port 10000 expecting https / ssl is this correct?