Watchguard XTM 11.6.1 VPN Authentication Failure
I have found XTM 11.6.1 (and some earlier versions) of XTM OS sometimes loses authentication mechanism on VPN SSL configuration.
I could not find the exact match when this happens, but under determined circumstances XTM OS loses the authentication mechanism for VPN SSL. Technically this turns into unchecked box on Authentication Server in VPN SSL settings, under the Authentication tab.
Although there is no workaround for this until next patch by Watchguard, if you use a monitoring system such as Nagios you might consider implementing a measure to verify if the authentication mechanism works. I have been trying to find a way to monitor the behavior of the authentication portal and found it can be monitored with an alteration of one of my previously created Nagios check scripts that is now available here.