I just add my answer because it could help someone else.
I had the same problem on a IAP-205, configured for a Guest Wifi only with Facebook login.
Users could connect to this free wifi and load the Facebook login page, but after entering the bypass code they received an error page: "Failed to authenticate client token. Please try again".
I had a remote assistance with the technical support without success, then I decided to update the IAP firmware and DRT directly from its UI.
After this update the login process started working again!