2
Do you/they have Norton Internet Security? Personal Firewall can prevent registrations on XOOPS based sites.
You might want to try customizing the Personal Firewall, or disable it all together when signing up. You will also have to disable it when accessing the Administration Control Panel.
If you don’t want to go a moment unprotected, you can always operate with 2 firewalls, using something like Zone Alarm (free) as the backup firewall when you have to disable Norton Internet Security’s Personal Firewall for any reason.
Hope this helps.