can't add sims contact error 400

I have seen it too but my suspicion is that it occurred when I didn't log into one of the social media accounts. Interestingly, I logged into all 4 accounts(game center/twitter/fb/googleplay) and never had it happen. When I switch to my ipad from the phone, 90% of my friends list is empty because I only logged into facebook. When I logged into twitter too, the rest of my friends appeared. Anyway, I suspect it happens when the game tries to add a player that uses an cloud account that you aren't using. I could be wrong.