reasonable guess - but not in this case!
the client’s PC cannot possibly have the same selection of extensions as my PC and on the day I turned a few of mine off to test just that - some are a bit aggressive.
there was no similar notice on any other site visited (the notice only came up when running the Virtualmin Pro upgrade through package manager)
As I had already performed the upgrade on other servers (GPL) without this problem https://forum.virtualmin.com/t/package-update-fails/136534 I was actually far more suspicious of Chrome or Microsoft than I was of Webmin.
who knows what upgrades there were on those overnight.