Thats strange, what browser are you using on the iPad (Chrome, Safari, other..)? Also try using a different browser and let me know if the issue persists in other browsers or after a restart.
I've had similar Safari issues on forums I own. I use a different forum software but perhaps the solution may work with XenForo software as well. On your iPad try to clear your history, cookies and data (Settings->Safari). Keep in mind this will delete all saved logins and site data on your Safari app. Also ensure cookies are allowed on this website.