Hiya,
Going to have to disagree, given that turning the websafe/site checking feature off in the router resolves the issue and users can then access the website, would suggest otherwise.
I am the hosting provider and developer for the website, the SSL failure message above is sent from the client device. Nginx, the web server in use, is used by millions of servers worldwide without issue, it simply cannot decipher the SSL data sent from the client when websafe is on. Looking over the internet, have this website from a VPN provider for instance, which shows 3 different blocking methods: https://bestvpn.org/virgin-media-blocks-vpns/. We're seeing the second one, not the first, no VPN is in use by those who are suffering the issue just to confirm that.
The chap I asked who uses Virgin and has the issue did the following: "I disabled a Virgin site checker thing on the router, might be virus checker, and it started to work.". I then asked the other person in the business who has the same issue, she did the same thing, and she could then access the website. Obviously, this is not tenable asking everyone who has this issue and actually contacts customer services (the vast majority will not), to turn the feature off on their router, reboot it, etc.
Thanks