I have this same issue when using Safari on mobile devices. Safari on desktop works as intended. I would be happy to help test, if there’s anything I can do.
Safari on my OSX/macOS desktop stays logged in. But Safari on iOS phone and tablet always makes me log back in again after what seems like about 2 hours of inactivity. If I use ICM to find a movie, I often have to log back in when the movie ends. And when I log in, I get the same “Whoops! Your form session has expired.” behavior described above, where I’m actually logged in but it looks like I need to log in a second time.
I don’t have any similar issues with any other sites. Two people I know in person say ICM behaves this same for them on iOS devices. My OS and browsers are updated, and set to accept all cookies. “Private mode” is not enabled. I am not using any extensions or ad blockers. I have tried unchecking “remember me,” clearing cache/cookies/data, then retrying later with and without “remember me.” I have tried Live and Beta, and I think they behave the same (though sometimes I’m forwarded from one to the other, so I may be wrong about that).
Since the issue is specific to mobile for me, I tested whether I was getting logged out when I locked my devices, restarted my devices, or killed the browser process. None of these recreated the issue; I stayed logged in after doing all of these. I wondered if the way iOS handles background processes could be causing the browser to appear as a new instance after some time. But if this were the case, I would expect killing the browser process to recreate the issue.
Let me know if there is any way I can help. I hope maybe my difference between Safari desktop and Safari mobile could help narrow it down to some session or cookie parameter that is handled differently between the two, and other browsers in this thread.