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
On 12/29/2015 at 10:05 AM, Clover13 said:
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.