The "password protected nodes" plugin is only compatible on xF 1.4x and half of it functions properly. You can unlock the forum no problem, however when you go to unlock the thread with the password it says "password confirmed, redirecting". It then redirects you to the page asking for the password again and will repeat that loop without ever unlocking the thread. I believe a few simple lines of code need to be tweaked, however I don't know jack about XenForo. If you're interested pm me.