The issue has been when connecting from any of my Apple devices, including my family devices. Only when through my VM router (whether direct or through the Time Capsule I have), and not when slaved to e.g. a mobile. Funnily enough, it started working last night for a while but has since stopped.
The error message is from Apple and very generic.
I've not had an issue before a couple of weeks ago. My wife and I have separate iCloud accounts and have the same problem.
Complicated I think but would welcome your thoughts.
I've had the same issues since Saturday (21st Jan). I was on the phone to Virgin Media tech support for an hour and a half last night and spoke to 3 different people with no resolution. They don't seem to know what the issue is and blame Apple.
I have spoken to Apple Support too but as it is an isolated issue on the Virgin Media network, they can't identify how to resolve it.
I have tried access iCloud Mail and other iCloud services using an iPhone, iPad, iMac and a PC browser. None of them are able to access using my Virgin Media home wifi network. When I am at work on a different wifi network, there is no issue. When I am on 4G there is no issue.