Apparently this is a known issue with a fix. I'd appreciate it if the site admin would fix this. I can't contribute while on the go (when I do most of my social media). I'm sure others would like it fixed too.
From the IP.Board Forum
Issue with iPhone not being able to sign in. Sign in process occurs, but on success, users still aren't signed in.
I moved iPhone/iPad user agents to the top and re-cached all. That fixed it.