We also have the same bug repported. Some users of iphone and ipad can’t login with Twitter.
It seems for some user that the login fails when they use and incorrect username/pw the first time. If that happens, there is a redirect to a not found page, and afterwrads it is not possible to login anymore.
We have a life website and we have had to remove the Twitter login!
Please check this issue asap.
Zanx