Please let this be the "official" thread, for reporting this issue so it be properly tracked!
I too, have been having issues logging into the website on my cell phone - it's inconsistent and I'm not sure what's causing the problem. In the meantime, I'm requesting that people who are having problems report the browser that they're using to connect with.
For me, I use the Dolphin browser fairly exclusively and have never been able to login with it. If I use Chrome on my phone however, it works fine.
Dolphin - Broken
Chrome - Good