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.