I resolved this problem today, and thought posting might help a few others. A search does not suggest it has been raised before.
An error screen with the above message started to appear this morning when I tried to log into my Sky email account. It was the same on my wife's laptop. However, the Bluemail app on my smart phone was still receiving incoming emails from the account.
It turned out that my IP address was the problem. It was in the form 51.6.*.*. After turning off the router and leaving it for a while (Sky suggested 30 minutes), a fresh IP was allocated in the form 146.199.*.*.
I can now access the website and my email account without difficulty. Sky were a great help, but why is the 51.6.*.* being blocked?