I had same problem in 7.0. Found out, I accidently changed my site URL in the Admin > Perference > Site URL name. Members got the E-mail from my site; but. they couldn't activate the account at my site. They got a server NOT FOUND error. I had typed in the block (which is to the site} http://simzoom.net as the URL. Visitors were on my server and could get to my phpNuke site. They could register and received an E-mail, But couldnt send me a activation code from the E-mail back to my server. What the Site URL needed to be is http://www.simzoom.net/phpnuke. After setting the right Web Site Configuration, members can activate their account.