If all these are as you say... Are you sure you're not on the user auth
portal instead of the management? Really you've already covered
everything, most probably, I'd detach the device from the network
(hopefuilly HA), grab my backup and just reset t...
Log into the device's GUI via the same subnet (you can NAT if that's the
only way). In the GUI navigate to system settings and check the allowed
login subnets. By default the FAC on those versions don't allow you to
log in if you're outside the subne...