Try disabling any adblock program/addon your using.

Few days ago i decided to try adblock plus (addon for firefox). When i have it activated the ffxiv launcher is acting really weird. It takes ages to load the boxes in which you enter your login info and things like that.
Just today it gave me the error message this topic is about. Since the only change to my system revently was installing the adblock i turned it off and tadaaa: launcher works fine again.

hope that helped

cheers Ty