It's windows 10 firewall. After sending too many Http requests windows firewall detects it as a security matter and blocks any further requests. If I try from any other non-windows device, the page loads and windows will keep show the error message, which ever browser I used.
Sometimes, loading another arduinow webserver address works but fixes the issue only momentarily. Nothing arduino based and network related will be allowed for minutes, even hours or until I restart my computer.
This bug has been going on for months now since Windows 10 came out and nobody has ever been able to find a workaround. I can't find any forum dealing with it seriously because everybody goes with it while it's a total absurdity. Since it's always temporary, I suppose nobody at Microsoft will ever deal with it...