I got that "untrusted" message today when my router was re-connecting.
So far it has disconnected and reconnected four times today.
Once the router gets its connection back, the web browser doesn't give the "untrusted" message. In this case, it had to do with not completing the authentication when the the router disconnected -- nothing to do with the website actually being "untrusted." It's just that when the router disconnects at the same time you're trying to get a webpage, it can partially get through but fail the authentication.