Certain users can't log in on iPad
We're seeing the weirdest thing on our 2.1.11-powered forum, forum.audiob.us: two users have reported that they're unable to sign in on their iPads, but can sign in just fine on their desktop machines. They tell us that after they've put in their login details, they are redirected to the front page of the forum again, but are still not logged in (they just see the "Welcome, Stranger" message again).
We can't reproduce the issue at all, and no one else has reported problems.
Getting the users to clear their cache seems to have no effect.
I've taken a look at the server logs during one users' login transaction, and it just looks normal: a GET to /entry/signin, a POST to /entry/signin (200 response), a GET to /discussions (200 response). But then, I'm seeing him going back to the signin screen.
We're using BotStop, Akismet, Tapatalk, Stop Forum Spam...nothing else of particular relevance, as far as I know.
Any ideas about where I can start looking for a solution?