After a recent update on a WatchGuard XTM 5 series we were unable to login using Internet Explorer 10 and the WatchGuard Web UI. The only way to connect to the WatchGuard XTM firewall was to use the WatchGuard System Manager. For us this a bit of a handicap in several ways and we refuse to use Chrome or Firefox since they tend to be less secure.
We finally were able to fix it with a suggestion from WatchGuard support.
Lastly, verify that the system has resolved the certificate issue by attempting to log into the Web UI again.
Now is also a great time to verify that the NTP settings are enabled which will affect the login process too.
We hope that this information is helpful. Please let us know how this has helped you or if you have additional questions. As always Firestone Technical Resources, Inc. is here to help with your computer support issues - "Providing personal service for your impersonal technology."