I've been having the same problem for about a week. I always tried with my email though so that funds were sent to my Xapo wallet. I just used my XAPO BTC Address instead of my email and it worked. I signed out and tried my email address to login and it still doesn't work. I logged back in with my XAPO BTC Address and it worked again. So for now, I suggest you use your BTC Address.