"Over the course of the past year, we have had a number of people who could not register, with the forums behaving as if they were not allowed to make cookies.
One of my forum members figured out what was going on. Apparently, some firewalls will not allow the creation of the cookie for getting past the acceptance screen. The solution to this is for the person to turn off their firewall, subscribe and turn it back on immediately. This has worked for about a half dozen folks on my forums so far who have had this problem.