Originally Posted by
Pontius Navigator
The failed log in screen has exactly the same alpha numeric URL as on the devices that log in OK.
That's not a 100% proof! There are some circumstances when this misleads you. Yes it's unlikely, but if different browsers are affected on only one device, there might be a redirection malware as a common reason. picks up your URL and redirects it to another IP address while still showing the same URL.
I have some other ideas but not a good one.