It works fine on mine, so it's not the site. Are you using the most up-to-date iOS? How are you reading, are you using safari, or one of the apps? If you're on Safari, try switching to the mobile site (or desktop if you're already on mobile) and see if that helps.
Otherwise try asking in the Proboards Support Forum as it's a technical issue, not really anything I can help with I'm afraid.
I have a 1st gen iPad that I usually use when I'm on here and it crashes / logs me out all the time too. I suspect it is the iPad and not the site since I can't update the iOS on it to the most current version (the older hardware is no longer supported). So if you're using an older one too, that's probably what's going on.