I tried signing in via Tapatalk, still not working. And yes, it does work with other XenForo sites. I've seen it, so that's not an excuse.
I've said this, haven't I? Tapatalk Compatibility is pretty far down on our list of things to do. Higher priority is getting the radios sorted, providing regular content for the site, keeping on top of updates and coming up with some new themes for the site. I'm not saying it won't ever be done, I'm saying it's going to be a while. For now, please be patient. It's low priority because the site works fine on mobile devices as is. Tapatalk was mainly added for when that wasn't the case and we had nothing we could really do about this. Today, we have Bootstrap. That makes squeezing things into smaller screens is far easier than when the last version of Everypony was created.