No sure if this is a Webmin or Virtualmin issue ... On the system information page (be it from the webmin or the virtualmin tab) if I open the "server status" accordion and click the small icons to restart a service (for example Postfix but it's the same whatever the service), the service seems to be restarted but the right side of the screen becomes grey and stays like this. I still have the left-hand menu so I'm able to go back to that page.
I see this on 9 different vps's, all with Debian from Debian 6 up to 9, all up to date:
Webmin version 1.872 Usermin version 1.732 Virtualmin version 6.02 Theme version Authentic Theme 19.07
And I am pretty sure this was not the case before but I cannot really says when it started ...