We had this occur in our environment before with SAML configured to utilize IWA:
_WebUIAppEnv_SAML_AUTHNCONTEXT=urn:federation:authentication:windows
Turns out within the Personal Certificate store for the Service Account our BigFix Root Server service ran under had some smart card certificates propagate to the store. Removing these certificates from the Personal Certificate store for the Service Account that ran our BigFix Root Server service fixed this for us. May or may not be the case here but worth a shot.
May want to disable automatic smart card certificate propagation on your servers also just in case.